The Sedevacantist would have us believe that a pontiff can lose his office by professing "manifest" heresy or committing public acts of apostasy. Yet in such statements there is usually lacking a distinction on what constitutes a "manifest" heresy or a "public act" of apostasy. As we shall see, there is not a direct correlation between holding or teaching a heretical proposition and being a "heretic", nor between committing an act of apostasy and being an apostate in the formal sense. We shall explore this distinction with a lengthy examination of one of history's less memorable pontiffs, Marcellinus, who was pope during the Great Persecution of Diocletian. Marcellinus is of great interest to this discussion because he very well may have committed a public act of apostasy.
In the Liber Pontificalis, we find the following entry under Pope Marcellinus (296-304):
Marcellinus, by nationality a Roman, son of Projectus, occupied the see 8 years, 2 months and 25 days. He was bishop in the time of Diocletian and Maximian, from July I in the 6th consulship of Diocletian and the 2nd of Constantius [296 A.D.] until the year when Diocletian was consul for the 9th time and Maximian for the 8th [304 A.D.]. At that time was a great persecution, so that within 30 days 17,000 Christians of both sexes in divers provinces were crowned with martyrdom. For this reason Marcellinus himself was dragged to sacrifice, that he might offer incense, and he did it. And after a few days, inspired by penitence, he was beheaded by the same Diocletian and crowned with martyrdom for the faith of Christ in company with Claudius and Cyrinus and Antoninus.
Another manuscript of the Liber Pontificalis contains an addendum to this episode, relating the penance and restoration of Pope Marcellinus and his eventual martyrdom at a local synod at Sinuessa in Italy:
And after a few days a synod was held in the province of Campania in the city of Sessana [Sinuessa], where with his own lips he professed his penitence in the presence of 180 bishops. He wore a garment of haircloth and ashes upon his head and repented, saying that he had sinned. Then Diocletian was wroth and seized him and bid him sacrifice to images. But he cried out with tears, saying, 'It repenteth me sorely for my former ignorance,' and he began to utter blasphemy against Diocletian and the images of demons made with hands. So, inspired by penitence, he was beheaded.
The tale is repeated in another document, the Passio Marcellini, which was written during the pontificate of Pope Symmachus (498-514). The Passio tells of the apostasy of Marcellinus and purports to contain some of the proceedings of the Synod of Sinuessa. In the record of the Passio, Marcellinus admits his apostasy and asks the synod to pass judgment on him. The synod, however, refuses to enter into judgment on the pope, stating that prima sedes a nemine iudicatur ("The first See is judged by none").
How does this episode bear on the Sedevacantist position? For here we have an instance of a pope guilty not of public heresy but of a public act of apostasy, which is worse. We also have the tale that when admitting his apostasy, a synod of bishops called to pass judgment upon him refused to do so. Several things can be affirmed from this episode, which we will tease out as we work out all the various facets and implications of the Pope Marcellinus affair.
Apostasy and Loss of Office
It is taken for granted in Sedevacantist apologetics that a pope guilty of heresy ipso facto loses his office. Granted what we have in the case of Pope Marcellinus is not heresy but apostasy. Nevertheless it is commonly asserted that a public act of apostasy would result in loss of office, since apostasy, like heresy, cuts one off from the Church.
Let us establish this point a little further. The Catechism of the Catholic Church notes that apostasy, like heresy and schism, is a sin that "wounds the unity of Christ's Body" . When a sin is said to "wound" the unity of the Church it is another way of saying that the commission of such a sin puts one outside the communion of the Church. This is self-evident with apostasy, since the definition of apostasy is "the total repudiation of the Christian faith" . The Catholic Encyclopedia defines apostasy as the "voluntary abandonment of the Christian religion", . The Catholic Encyclopedia distinguishes this sort of apostasy, called apostasy a fide, from other sorts of apostasy, such as when a religious abandons his monastery or a cleric abandons the duties and dress of the clerical state.
Pope Pius XII in his encyclical Mystici Corporis Christi stated:
Not every sin, however grave it may be, is such as of its own nature to sever a man from the Body of the Church, as does schism or heresy or apostasy. Men may lose charity and divine grace through sin, thus becoming incapable of supernatural merit, and yet not be deprived of all life if they hold fast to faith and Christian hope, and if, illumined from above, they are spurred on by the interior promptings of the Holy Spirit to salutary fear and are moved to prayer and penance for their sins .
Thus apostasy, like heresy and schism, "severs" one from the Body of Christ. In the case of a Roman pontiff, does this imply a loss of office? Sedevacantists have frequently argued that the profession (or sometimes the mere private belief) of heresy on the part of a Roman pontiff implies that the heretical pope ipso facto loses his office. The same would obviously hold for apostasy as well.
This would seem self-evident, given the teaching of Pius XII. Leaving aside for a moment the question of heresy and the papal office - of which theologians are divided on - it would be difficult to argue that apostasy does not entail loss of office, inasmuch as apostasy is a much more serious sin than heresy. Heresy is merely the rejection or denial of a particular point or points of Christian doctrine, but apostasy is the rejection of the entire Christian faith. Regardless of what position one takes on the heresy question, is it unfeasible to see how a cleric could retain their office if they renounce the Christian faith entirely. There is thus a sound a fortiori argument that apostasy entails loss of office.
But beyond this, there is sound canonical precedent. In the famous Decretals of Pope Gregory IX (1234), it is declared that clerics guilty of apostasy lose all all dignities, offices, benefices, and every clerical privilege . Thus we have clearly stated canonical statute affirming that a cleric guilty of apostasy ipso facto loses his office. The text of the Decretals does not note any exception for Roman pontiffs.
Thus, it seems beyond a reasonable doubt that it is a sound proposition to affirm that an apostate cleric thereby loses his office in the very commission of his apostasy.
What is the Liber Pontificalis?
Before we start drawing unwarranted conclusions, let us examine the Liber Pontificalis, where the tale of Pope Marcellinus' apostasy is told.
At its core, the Liber Pontificalis ("Book of the Popes") is an alleged book of papal biographies of all the popes dating from St. Peter to about the late 9th century. Later additions and amendments were made to bring the list up to the 15th century, but the original work only goes up to Pope Stephen V (d. 891). Though beginning with St. Peter, most scholars agree the work was written in the 5th or 6th century. Several editions were in circulation, the earliest dating from around 537. By the way, Evolution Publishing (which is a subsidiary of Arx Publishing) has an excellent little edition of the Liber Pontificalis in their Christian Roman Empire series. In the interest of full-disclosure, I should mention I also have a book on St. Cyprian available through Evolution Publishing with an introduction by Ryan Grant. But I digress.
Most scholars also agree that the authors of the Liber Pontificalis were apostolic librarians or notaries of the Church of Rome, most likely attached to the papal court, who wrote for the purpose of advancing the spiritual and temporal claims of the Church of Rome. It was cited uncritically from the Carolingian era until the 1700s, serving, as many have noted, as one of the premiere piece of papal propaganda in the Holy See's many power struggles over the centuries.
The historical value of the Liber Pontificalis varies. The biographies up until the reign of Anastasius (496-498) are historically unreliable, consisting of little more than a mixture of history, tradition, and fable that collectively constitute the legendarium of the Church of Rome's early history. The Catholic Encyclopedia calls the early biographies "full of errors and historically untenable" , and in this estimation most scholars seem to be in agreement. The biographies from 500 through the Carolingian period, however, are generally considered to be historically reliable, as the authors had access to more contemporary sources.
The value of the Liber Pontificalis, then, is not so much that it is an accurate history of the early Church at Rome as much as it is a reflection of what the early medieval Roman Church believed about its own history. The Liber Pontificalis, at least the entries up to Pope Anastasius, tells us much more about what the 7th and 8th century Roman Church believed than what actually happened in the first four centuries.
Did Pope Marcellinus Really Apostasize?
Of course this unreliability of the first half of the Liber Pontificalis has led many to question whether Marcellinus really apostasized at all. The episode narrated in the Liber Pontificalis was written at a minimum of 250 years after the fact, and most likely more; given the admittedly poor historicity of the Liber Pontificalis, could the whole episode have been a fabrication?
Evidence Against Marcellinus' Apostasy
There are a couple of facts put forward as evidence that Marcellinus never truly apostasized. For one thing, outside of the Passio Marcellini and a single manuscript of the Liber Pontificalis, there is no mention of any Synod of Sinuessa. In other words, there are no contemporary sources that reference the Marcellinus episode. Most Church historians believed the meeting at Sinuessa never happened.
There is also no reference to the apostasy of Marcellinus in any contemporary documents. Surely had the pontiff apostasized, his enemies or representatives of various heretical sects would have publicized the fact?
Finally, as we have noted, the alleged minutes of the Synod of Sinuessa that are found in the Passio Marcellini did not appear until the pontificate of Pope Symmachus (498-514). It just so happens that the pontificate of Symmachus was torn by a schism between himself and one Laurentius, a bishop installed and supported by the Byzantine Emperor Anastasius I. Symmachus had been accused of various crimes by his rivals and was summoned to defend himself in a synod ordered by the Ostrogothic King Theodoric in 501-502. Initially compliant, Symmachus subsequently refused to participate in this synod because of violence against members of his entourage, as well as the presence of a visiting bishop in the city of Rome, which suggested the See was vacant. Even so, the bishops of the synod came to the conclusion that they could not pass judgment on Symmachus, since he was the successor of St. Peter, and commended the affair to God. Thus Symmachus was entitled to the full exercise of his episcopal office.
It was during this contest that the alleged minutes of Sinuessa appeared, containing the principle prima sedes a nemine iudicatur ("The first See is judged by none"). Thus it is plausible that a propagandist of the cause of Symmachus produced the forged minutes to lend strength to Symmachus' refusal to submit himself to the judgment of the synod. It is noteworthy that the principle upon which the synod of 501-502 exonerated Symmachus - that they had no authority to judge him because he was the successor of St. Peter - is the same which was allegedly put forward at Sinuessa: "The first See is judged by none." It could be possible that the decrees of Sinuessa were forged by a partisan of Symmachus to create a precedent for this in the life of Pope Marcellinus.
It is believed that the Passio Marcellini, with its account of the synod of Sinuessa, served as the source for the text of the Liber Pontificalis.
Thus, the evidence against the apostasy of Marcellinus boil down to two arguments: an argument from silence (as there are no contemporary documents attesting to the apostasy from the time of Marcellinus), and casting a cui bono ("To whose benefit?") apprehension upon the narrative of the Passio Marcellini and hence the Liber Pontificalis.
Evidence For Marcellinus' Apostasy
Nevertheless, the argument is far from conclusive. As we know, arguments from silence are only of so much value, as are cui bono arguments; at best, such arguments can support a thesis when additional evidence is present, but not in lieu of such evidence. The fact that a son inherits money when his father dies is alone insufficient to establish that he in fact killed his father. Similarly, the fact that the judgment of Sinuessa supported the position of Pope Symmachus is insufficient to establish that the party of Symmachus actually forged the documents of Sinuessa. Other evidence would need to be forthcoming.
Furthermore, the skepticism of the accounts of the Liber Pontificalis and Passio Marcellini center on the question of the historicity of the Synod of Sinuessa, not the apostasy of Marcellinus. Even if Sinuessa never happened, that would not constitute an argument that Marcellinus never apostasized.
Thus the argument that Marcellinus never apostasized remains a conjecture - a conjecture not without some plausibility, but still a conjecture. Is there evidence to the contrary, that Marcellinus did apostasize? As a matter of fact, there is.
To begin with, the Roman Chronograph, a kind of early Christian calendar containing a list of consuls and reigning popes up to 354, omits any mention of Pope Marcellinus. A similar calendar issued two decades earlier, in 336, also fails to mention him. The Martyrologium Hieronymianum, an early Roman martyrology, does not mention Marcellinus, despite the fact that the Liber Pontificalis - and Roman custom - has him suffering martyrdom. Taken collectively, it is difficult to see how these omissions were not intentional.
Eusebius mentions Marcellinus in his history, but says only cryptically that "the persecution also affected him"  but does not say he was martyred. In what sense was he "affected"? It has also been noted that in the entire persecution of Diocletian we know of only two members of the Roman clergy who were martyred, and both belonged to minor orders. Neither the pope nor any of the higher clergy suffered death. In a persecution as intense and protracted as that of Diocletian, how is it possible that the most prominent ecclesiastic in the world escaped death? Perhaps he hid himself or went into exile, as St. Cyprian did during the persecution of Decius. However, it was not Marcellinus alone, but the entire upper echelons of the Roman clergy who escaped.
Is it plausible that in the entire episcopate and diaconate of Rome eluded Diocletian? Or is it possible that somehow Marcellinus was able to procure immunity for himself and for the Roman Church? This could have been by formal apostasy, or perhaps by some compromise, such as bribery or the purchase of a certificate (libellus) of sacrifice. The latter seems unlikely, however, since if Marcellinus had merely obtained a letter of sacrifice without performing the act, we can presume he or his companions would have made this known once the persecution ended so that the pope did not go down in history as an apostate. Therefore it is more likely that some sort of sacrifice was made, whether a pinch of incense, a libation or otherwise.
This is mere conjecture, of course, though it is certainly a valid question to wonder how Marcellinus and his entire entourage escaped the most intensive persecution in the Church's history. Perhaps they did not escape; perhaps he was arrested and apostasized under fear.
This seems even more plausible when we consider that rumors of the pope's apostasy had spread by the late 4th century. The Donatist bishops of North Africa, in arguing that the mainstream Catholic clergy were corrupt and weak, cited the apostasy of Marcellinus as evidence. A letter written by the Donatist Petilianus around 400 accuses Marcellinus of not only offering incense but of being one who handed over the sacred books (traditore). Similar rumors were circulating around Rome, as well. These rumors of apostasy were circulating by the late 380s, several centuries prior to the Passio Marcellini or the Liber Pontificalis.
Those who disbelieve in the apostasy of Marcellinus tend to cite a letter of St. Augustine in which the great bishop of Hippo dismisses the charges against Marcellinus as fabrications. However, a close reading of Augustine demonstrates that this is not the case.
In his Answer to Petilian, Book II, Petilian accuses Pope Marcellinus, and several other Roman clergy, of offering incense during the persecution and offering up the sacred books. This is supposed to be part of a larger argument that the Catholic Church's rapport with the Christian Roman Emperors has made her complicit in the sins of the state. It is an attack on the very legitimacy of the Catholic Church. Augustine responds by saying:
For now you go on to make mention of the bishops whom you are wont to accuse of having delivered up the sacred books, concerning whom we on our part are wont to answer: Either you fail in your proof, and so it concerns no one at all; or you succeed and then it still has no concern with us. For they have borne their own burden, whether it be good or bad; and we indeed believe that it was good. But of whatever character it was, yet it was their own; just as your bad men have borne their own burden, and neither you theirs nor they yours. .
The attentive reader will note Augustine does not absolutely deny the truth of the charge. He says merely that Petilian offers no proof for his accusation, though Augustine personally believes it to be false; but more than that, Augustine says that even if the charges were true - even if Pope Marcellinus did apostasize under duress - "it still has no concern with us". In other words, whether or not this particular pope offered incense under duress is neither here nor there as far as the legitimacy of the Church is concerned. Just as Catholic apologists today argue that the existence of bad popes like John XII or Stephen VII does not disprove the doctrine of papal infallibility, so Augustine argues that whatever burden Marcellinus may have succumbed to in his day is of "no concern" to Donatist arguments against the holiness of the Catholic Church. The deeds of bad men do not negate the witness of good, whether they be Donatist or Catholic.
Ultimately, Augustine is familiar with the charges against Marcellinus and personally believes them to be untrue but is not absolutely certain - hence his qualifications. Given that Marcellinus was omitted from two practically contemporary pontifical regnal lists, that Augustine's defense of him was very qualified, and that the pope seems to have escaped the persecution completely unscathed, it seems very plausible to say that this pope was involved in something unsavory during the great persecution. The only explanation ever given as to what this unsavory activity was is that it was apostasy, as noted in the Liber Pontificalis, Passio Marcellini, letter of Petilian and the response of Augustine.
While we do not state that his apostasy is certain, we can nevertheless say that there are probable grounds for assuming that there is a core of truth to the story of the apostasy of Pope Marcellinus.
A Plausible Sequence of Events
At this point you may be getting impatient and wondering whether Marcellinus did or did not apostasize. The truth is, we cannot be certain. Like St. Augustine, we may have our own opinion in the matter, but there is no way to be sure. He may have offered incense to the pagan gods under duress, or he may not have. Both possibilities are plausible.
The fascinating thing is that the Church of Rome thought so as well. We mention that the story of Marcellinus' apostasy is mentioned in the Liber Pontificalis. It is astonishing that the Liber Pontificalis - which is supposed to be a work of pro-papal propaganda asserting the prerogatives of the Holy See - should include the account of Marcellinus' apostasy. Its inclusion does not mean it actually happened, but it does mean that whoever compiled the Liber Pontificalis believed it actually happened. This would be the only reason why the 7th century compiler of the work would include this embarrassing episode.
And this is really the crux of the matter, that the Roman Church believed Marcellinus had apostasized. This much is undeniable. What is truly fascinating is how the Roman Church and the successors of Marcellinus sought to deal with this story.
There is one contradiction we have not yet explored: the Passio Marcellini and the Liber Pontificalis, as well an early Roman tradition, all attest that Marcellinus died a martyr. Yet his martyrdom is not mentioned by Eusebius, nor in the Martyrologium Hieronymianum, nor in the Roman Chronograph of 354, nor in the martyrology manuscript known as Codex Bernensis, even though each work contains the lists of many other martyr-popes (he does appear in the martyrology of St. Bede, though Bede was merely drawing on the text of the Liber Pontificalis). The burial location of Marcellinus was venerated in the early church, but subsequent excavations there have failed to identify any grave belonging to him, very unusual in the case of a martyr-pope. Why such ambiguity surrounding the nature of his death?
I would like to hypothesize the following sequence of events that may tie all these disparate pieces together:
At the outbreak of the persecution of Diocletian, Pope Marcellinus is arrested and summoned to sacrifice to the pagan gods. Under duress or perhaps some torture, he and perhaps some of his closest high-ranking clerics break down and agree to pinch incense to Caesar. Of course, he does not intend to renounce Christianity; his act is one of momentary weakness, prompted by torture or fear.
The magistrate, as was often the case in Christian persecutions, probably was not really interested in hunting down Christians; he only had to make a show of it, get a few high-profile sacrifices under his belt to prove he had done the emperor's will. He tells Marcellinus that if the pontiff will just pinch some incense he will conveniently ignore the rest of the Roman hierarchy. This is further inducement to sacrifice, and also explains why so few of the Roman hierarchy perished in the following years.
The persecution of Diocletian is a very chaotic time. Presumably Marcellinus does not will to repudiate the Christian faith and repents in some manner. He dies naturally, however, before the persecutions ends (he died in 304). By the time the dust has all settled and Christianity is legalized, rumors start to spread about the conduct of the late pope during the persecution. The clergy and people of Rome, those who knew Marcellinus and the truth of what occurred, are deeply embarrassed by the incident. Some of the documents compiled in the next few decades omit his name from the papal lists out of shame, a practice not unknown in the ancient world when the memory of a ruler was shameful or embarrassing. 
Known in Rome, rumors of the pope's less than exemplary behavior spread throughout the empire, though the details were never quite known for certain. Eusebius knows that something shady happened. The Donatist heretics in North Africa get a hold of the story in the 380s, though by then it is embellished considerably; Marcellinus not only pinched incense, but handed over the Sacred Scriptures, as did Melchiades, Marcellus and Sylvester, Marcellinus' three successors in the Chair of St. Peter. Augustine retorts that there is no way to prove the story, and while he personally disbelieves it, states that it is ultimately irrelevant to the argument made by the Donatists.
Another century goes by. The persecutions are long gone and the details quite muddled. A schism erupts in the Church between Pope Symmachus and the anti-pope Felix II. King Theodoric summons Symmachus to a synod of bishops in 501 to answer certain charges. Symmachus at first agrees, but after his entourage is ambushed while leaving the Vatican, he refuses, despite continued pressure from the king.
During this controversy, a partisan of Symmachus writes the Passio Marcellini, knowing the haze of uncertainty that surrounds this particular pontificate will provide a worthy template upon which to fabricate precedent that will support Symmachus. Hence he takes the well-known story of Marcellinus' apostasy and adds on an invented "Synod of Sinuessa", which refuses to judge Marcellinus on the principle that "The first See is judged by none." Knowing, however, that Marcellinus is not the best role-model for Symmachus to appeal to, the author of the Passio Marcellini turns Marcellus into a sympathetic character by creating a story about his repentance and crowning it with martyrdom. The Passio does its trick; the 501-502 Synod of Parma convened by Theodoric refuses to judge Symmachus, citing the argument that the successor of Peter cannot be judged by any earthly tribunal. Symmachus emerges victorious from the schism.
Another century and a half go by. By this time not only are the details of the pontificate of Marcellinus hazy, but even the facts of the schism of Symmachus are uncertain. Based on the text of the Passio Marcellini, which is still circulating, many Christians now account Marcellinus to be a martyr and are visiting his tomb. By this time (late 7th century), Rome is in conflict with the See of Constantinople over the question of primacy. As part of this long conflict, notaries of the Roman Church are commissioned by some pope to go through the records of the Church of Rome and compile a comprehensive biography of the lives and deeds of the popes with an eye towards demonstrating the historical primacy of the Roman See. Thus is born the Liber Pontificalis.
As the author of the Liber Pontificalis goes about his research, he discovers the omissions and ambiguities surrounding the question of Marcellinus' apostasy. He also encounters the Passio Marcellini with its tale of Marcellinus' apostasy, repentance, martyrdom, and the minutes of the fabricated Synod of Sinuessa. He is, however, uncertain, as his research turns up no other evidence in the Roman archives of any Synod in Sinuessa. Knowing, however, that there is a strong tradition that Marcellinus apostasized, repented and was martyred, he includes the tale in the Liber Pontificalis but leaves out mention of the Synod of Sinuessa, which he finds questionable. Later redactors of the Liber Pontificalis, still uncertain but wanting to err on the side of caution, decide to include the minutes of Sinuessa. This explains why one manuscript includes the Synod of Sinuessa while others omit it.
From about the 8th century until the modern age, the Liber Pontificalis was taken at face value as an accurate account of the history of the early papacy. Thus, for over 1,000 years, Christian historians - including those of the Church of Rome - took the story of Marcellinus as presented in the Liber Pontificalis as historically accurate. The apostasy, repentance, and martyrdom of Marcellinus were looked upon as established historical facts.
Implications for Sedevacantism
What implications does this have for the sedevacantist? The first point is noting what the Church did when confronted with the possibility of Marcellinus' apostasy. The Roman Church, who as we have stated, assumed that Marcellinus did apostasize, did not attempt to suggest that Marcellinus thereby lost the papacy or fell from office. This point bears repeating: the Roman Church believed that Marcellinus had apostasized, but nobody suggested he lost his office. He was omitted from some episcopal lists in the mid-300s, but appears in all the later lists, and there is no evidence of an antipope during his lifetime. Clearly, everybody regarded Marcellinus as the legitimate pope, both before and after his apostasy.
The Roman Church did not deal with the issue of his apostasy by denying he held the papal office, but rather by turning him into a martyr to save face. Believing Marcellinus apostasized, the issue was not whether or not he lost the papal office, which nobody suggested, but rather how to spin the affair so that Marcellinus was redeemed in the end, and hence the tale of his martyrdom as narrated in the Passio Marcellini.
The Passio Marcellini also invented the Synod of Sinuessa as the context in which the principle prima sedes a nemine iudicatur ("The first See is judged by none") was first enunciated. Of course, the proceedings of the Synod of Sinuessa are a forgery. But the historical Synod of Parma in 501-502 affirmed this same principle when called upon to judge Pope Symmachus. Thus the concept of the immunity of the Roman pontiff from episcopal judgment passed into Canon Law. Hence we see Pope St. Nicholas I (858-867) stating to the Byzantine Emperor that "Neither by Augustus, nor by all the clergy, nor by religious, nor by the people will the judge be judged...The first seat will not be judged by anyone" . Pope Leo IX wrote in 1053 to the Patriarch of Constantinople that "By passing a preceding judgment on the great See, concerning which it is not permitted any man to pass judgment, you have received anathema from all the Fathers of all the venerable Councils..."  The principle was again enunciated by Pope St. Gregory VII in his famous bull Dictatus Papae, which was a collection of precedents regarding papal authority from the popes of the first millennium. There Gregory affirms that "That he [the pope] himself may be judged by no one" . This principle was consistently reaffirmed in the Middle Ages and has passed into the 1983 Code of Canon Law, which stipulates that no pope can be subjected to any kind of a trial. This is because he is beyond judgement. 
It is ironic that this cornerstone of papal canon law was first devised from the acts of a synod that never took place to deal with the question of the apostasy of a Roman pontiff. Still, the principle stands, as it has been reaffirmed countless times. Therefore, even were a pontiff to apostasize or fall into heresy, it is clear that there is no tribunal or authority that could depose the pontiff nor even declare him to be deposed. If so, then what are we to make of the citations from Mystici Corporis and the Decretals of Gregory IX, which all imply that a cleric guilty of apostasy would ipso facto lose his office?
We know that to hold an objective heresy and to be a heretic are two different things. Most of us have, at some point in our lives, believed something false. But to simply evidence a belief in a heretical proposition does not make one a heretic. A fundamental characteristic of heresy is always the pertinacity or contumacy with which the heresy is professed; it necessitates a kind of obstinate, stubborn refusal to be corrected. This is why St. Thomas, citing Augustine, says:
By no means should we accuse of heresy those who, however false and perverse their opinion may be, defend it without obstinate fervor, and seek the truth with careful anxiety, ready to mend their opinion, when they have found the truth," because, to wit, they do not make a choice in contradiction to the doctrine of the Church. Accordingly, certain doctors seem to have differed either in matters the holding of which in this or that way is of no consequence, so far as faith is concerned, or even in matters of faith, which were not as yet defined by the Church; although if anyone were obstinately to deny them after they had been defined by the authority of the universal Church, he would be deemed a heretic. This authority resides chiefly in the Sovereign Pontiff .
There is not a one-to-one correlation between believing an objectively heretical proposition and "losing the faith" or being a full-blown heretic. Similarly, since apostasy is defined as the rejection of the Christian faith, the fact that Pope Marcellinus may have committed a formal act of apostasy does not suggest that he repudiated or rejected the Christian faith. He suffered a momentary lapse due to weakness and fear. Just as there can be no true heresy without contumacy, so there can be no apostasy without a willful repudiation.
A classic example of this as St. Peter. Despite the fact that Jesus promised Him that his faith would not fail (Luke 22:32), we see at the arrest of Jesus that Peter not only disowns Jesus but denies Him "with an oath" (Matt. 26:72). Yet even here he did not "lose the faith." Losing the faith, becoming a heretic, becoming an apostate is something deeper than a momentary lapse. It is a deep condition relating to the rejection of God or the truths of the Catholic faith.
In Aquinas' Catena Aurea on the Gospels, the passage on Luke 22 where Jesus promises Peter his faith will not fail, we notice a distinction is made between the act of denying the Lord and a loss of faith. Thomas cites the authority of several other doctors:
Theophylact: "For albeit thou art for a time shaken, yet thou boldness store up, a seed of faith; thou the spirit has shed its leaves in temptation, yet the root is firm."
Chrysostom: "He does not say, 'I have prayed that thou deny not,' but that thou not abandon the faith."
To lose the faith is not the same as to deny. Thus, though it is always scandalous when a pontiff performs gestures damaging to the faith, or like St. Peter and Marcellinus, may even commit a public act of apostasy, we do not have grounds for assuming that the pontiff has "lost the faith", much less grounds to judge that he is deposed, since "the first See is judged by none." This is why St. Peter did not lose the faith, and why Marcellinus did not lose his office.
There is one more piece of evidence that complicates the puzzle: Marcellinus is a saint.
That's right. St. Marcellinus, with a feast day on April 26th. We mentioned that the Roman Church seems to have assumed he apostasized, but the facts surrounding his death have always been obscure. His commemoration was removed from the calendar by Paul VI in 1969 because of the uncertainties surrounding his papacy.
But, for over 1,000 years, the Roman Church venerated as a saint a pope which they believed committed a formal, public act of apostasy. And nobody raised any problem with it or suggested he lost his office.
 CCC 817
 CCC 2089
 (1907). "Apostasy." In The Catholic Encyclopedia. New York: Robert Appleton Company. Retrieved March 6, 2015 from New Advent:http://www.newadvent.org/cathen/01624b.htm
 Pope Pius XII, Mystici Corporis, 23
 Decretals of Gregory IX. V, title 7, ix, xiii
 "Liber Pontificalis." The Catholic Encyclopedia. Vol. 9. New York: Robert Appleton Company, 1910. 6 Mar. 2015<http://www.newadvent.org/cathen/09224a.htm>.
 Eusebius, Ecclesiastical History, VII. 32
 St. Augustine, Letter Against Petilian, Book II, 208
 The famous damnatio memoriae, practiced in Rome right up until the age of Constantine, who himself had declared a damnatio memoriae against the former Emperor Maximian after the latter's death in 311.
 Pope St. Nicholas I, Proposueramus quidem, Denz. 330
 Pope St. Leo IX, "In terra pax hominibus" to Michael Cerularius and to Leo of Achrida, September 2, 1053, Denz. 352
 Pope St. Gregory VII, Dictatus Papae, 19
 CIC 1556
 St. Thomas Aquinas, STh, II-II, Q. 11, Art. 2