In his 1947 encyclical Mediator Dei, Venerable Pius XII warned against what he termed an "exaggerated and senseless antiquarianism" in the reform of the Sacred Liturgy. This preference for "antiquarianism" has subsequently become known as archaeologism, and from the 1930s till today has been one of the most prevalent schools of thought in discussions about Catholic identity. What is archaeologism, why is it so prevalent, why did Pope Pius XII warn Catholics against it, and how does it promote a distorted view of the Catholic tradition? God willing, we will endeavor to answer these questions.
What is archaeologism?
Archaeologism is not so much a heresy as a trend, a certain approach to Catholic liturgy and practice. Its distinguishing characteristic is an excessive value placed on those Catholic practices which came earlier in historical-chronological succession. For the archaeologist, first is always best. A practice or prayer of the patristic Church is "better" or "purer" than a practice of the medieval Church. Consequently, the goal of any true liturgical renewal ought to be to return to the practice of the first Christians, inasmuch as possible. The modern Church ought to imitate the apostolic Church.
The origin of archaeologism lay in the liturgical movement of the 1930's. With growing questions about how to more appropriately engage the faithful in the Mass, liturgists posited, among other things, a return to apostolic liturgical forms as a means of promoting active participation. The thinking was that many gestures and rituals had accumulated over the centuries, and that these accumulations had become a barrier to truly engaging the faithful. These accretions placed undue obstructions between the faithful and God, and in many cases were completely unintelligible to the laity. Thus, in order to promote a true encounter with the living God, these accretions ought to be stripped away, back down to the simple, robust faith of the apostolic era, when liturgy was devoid of many externals and focused much more intensively on a personal encounter with the Risen Christ. Therefore, programs of liturgical renewal were most appropriate that would move the Church in this direction.
In fairness to the liturgical experts of the day, it must be pointed out that not even they envisioned a modern Church engaged in simple imitation. It is not possible to go back to the apostolic age, and the liturgical experts did not propose this. Rather, what they envisioned was elements of apostolic Christianity embodied in modern liturgical praxis - a sort of transplant, a process in which the spirit of early Christian thought and liturgy permeates our modern experience and leads to an encounter similar to that experienced by those first generations of disciples.
The Attraction of Archaeologism
Archaeologism is admittedly an attractive idea, and ostensibly a noble aim. Part of its attraction is that, at the outset, it appears to be very consonant with Catholic ecclesiology. The Catholic faith is a faith grounded in tradition. Tradition is the force that binds the Church's past with her present, and the content of the faith itself is safeguarded through the traditio, the handing on, which takes place in the context of a faithful passing on of the tradition to each subsequent generation. Catholic Tradition is nothing other than Catholic identity. As the Magisterium would teach at Vatican II, "The Church, in her doctrine, life and worship, perpetuates and transmits to every generation all that she herself is, all that she believes" . Given the Catholic emphasis on Tradition, why would we not want to return to our earliest roots? If tradition means adhering to what is handed on, what argument could anyone possibly have against going back to the sources, the well-spring from which this stream of Tradition takes its source? It would seem that for a Church grounded in Tradition, this would be not only a good thing, but even the most reasonable course of action. Archaeologism thus presents the appearance of great fidelity to Catholic ecclesiology, and as such attracts a great many Catholics who, though well intentioned, have perhaps only given superficial thought to the issue. As we shall see shortly, archaeologism is actually foreign to the spirit of the Catholic traditio.
The Warning of Pius XII
But Pius XII, while recognizing the need for greater engagement of the Christian faithful in the liturgy, saw a danger in this approach. While an appreciation for Tradition is essential for any authentically Catholic approach to worship, it must be recalled that our Tradition is a living Tradition. The fundamental danger in archaeologism is that it views the Church and her practices as an historical artifacts rather than a living, organic unity. In warning against archaeologism, Pius XII draws attention to the living nature of the Church:
"The Church is without question a living organism, and as an organism, in respect of the sacred liturgy also, she grows, matures, develops, adapts and accommodates herself to temporal needs and circumstances, provided only that the integrity of her doctrine be safeguarded. This notwithstanding, the temerity and daring of those who introduce novel liturgical practices, or call for the revival of obsolete rites out of harmony with prevailing laws and rubrics, deserve severe reproof." 
He calls for the Church to be understood as she is - a living organism, who like a person, grows, matures, and develops. Just as with a human being, each growth and adaptation the Church undergoes over the centuries becomes part of her identity. Both the things that shaped my own development when I was 12 as well as those that did so when I was 25 are equally part of my personality, and to deny one in favor of the other or to regard older developments are authentic while newer ones are simple "accretions" is really to deny the organic nature of my personhood. In living organisms, everything that the organism does or adapts to over time becomes part of its identity. To be sure, certain traits or events might be repeated or strengthened over time, and thus may become stronger or more prevalent parts of my identity, but if they are stronger it is not because of how early they occur, but because of the constancy with which they occur over time.
The Church, too, must be viewed in this manner, although, Pius notes, this is not to give occasion for pure novelty, which is always reprobated. Pius goes on to laud the desire to engage with Catholic Tradition, but warns that this does not mean primitive practice is to be preferred to those disciplines which came later. In chapter 62 of Mediator Dei, he gives some examples of liturgical practices of the early Christians that he does not believe the Church should return to:
"Assuredly it is a wise and most laudable thing to return in spirit and affection to the sources of the sacred liturgy. For research in this field of study, by tracing it back to its origins, contributes valuable assistance towards a more thorough and careful investigation of the significance of feast-days, and of the meaning of the texts and sacred ceremonies employed on their occasion...But it is neither wise nor laudable to reduce everything to antiquity by every possible device. Thus, to cite some instances, one would be straying from the straight path were he to wish the altar restored to its primitive tableform; were he to want black excluded as a color for the liturgical vestments; were he to forbid the use of sacred images and statues in Churches; were he to order the crucifix so designed that the divine Redeemer's body shows no trace of His cruel sufferings; and lastly were he to disdain and reject polyphonic music or singing in parts, even where it conforms to regulations issued by the Holy See" 
But beyond noting that Pius disapproved of the ecclesiology evident in archaeologism, what danger is there in the archaeologist approach? How is the archaeologist approach positively harmful? How is it a distortion of the true spirit of Catholic tradition?
How Archaeologism is Against the Spirit of Tradition
A little later in the encyclical, Pope Pius XII speaks of archaeologism (which he refers to hear as "exaggerated and senseless antiquarianism") as not only a misconception, but as something detrimental to holiness in a very practical sense. It "paralyzes" and "weakens" the sanctification of the soul that is effected through the liturgy, causing "grievous harm" to souls. The pope writes:
"This way of acting bids fair to revive the exaggerated and senseless antiquarianism to which the illegal Council of Pistoia gave rise. It likewise attempts to reinstate a series of errors which were responsible for the calling of that meeting as well as for those resulting from it, with grievous harm to souls, and which the Church, the ever watchful guardian of the "deposit of faith" committed to her charge by her divine Founder, had every right and reason to condemn. For perverse designs and ventures of this sort tend to paralyze and weaken that process of sanctification by which the sacred liturgy directs the sons of adoption to their Heavenly Father of their souls' salvation." 
How is archaeologism so positively harmful?
Archaeologism is harmful primarily because, in conceiving of the Church as an artifact that needs to be recovered, it ends up casting aside the legitimate developments within the Church that have arisen throughout the centuries. The Church is essentially viewed as a ship covered with crusty barnacles which need to be scraped off. In this "scraping off" process, many valuable elements of tradition are lost, elements which grew into the Church's organic structure and are part of her identity. Since the Church's ends are supernatural - the salvation and sanctification of souls - every legitimate development within the Church contributes to this end. When legitimate developments are discarded as mere "accretions" or treated as extrinsic to the Church's identity, the Church is weakened in her ability to carry out her divine mandate.
A primary example is the liturgy. Nobody denies that the most ancient Christian liturgies were probably simple affairs, at least more simple than we see in the high middle ages. Though this may be true, the Church, in her wisdom, gradually added more elements to the liturgy for the purpose of glorifying God and highlighting the sacrificial nature of the Eucharist. This was done to aid the faithful in learning the truths of the faith (finis paedagogia) as well as appropriating the grace of God (finis sanctitatis). Thus, to simply cast aside legitimate developments in tradition is to diminish both the pedagogical value of the Mass as well as the ease with which the faithful can appropriate God's grace ex opere operantis. This is why Pope Pius XII says that an archaeologist approach to liturgy weakens the process of sanctification. It simply closes off many great treasures of the Church to the faithful.
Ultimately, archaeologism forgets Christ's words that He will be with the Church for all time, and that the Spirit will abide with the Church to lead it into all truth.  For the archaeologist, the Church is not a living thing whose developments are all part of its very identity; it is something that was founded by Christ and then left to the wiles of history, similar to how the Deists view the creation of the world. Adherents of archaeologism may deny this, but it amounts to the same thing in practice.
However, if, as Pius says, the Church is a living thing who is guided in its growth by Christ through the Holy Spirit, no legitimate development, liturgical or otherwise, can ever be discarded as positively harmful to the faith. Traditions may certainly die; many have done so in the past, but in an organic manner, as occurs with living things. It is one thing to note that branches of a tree have withered; such branches may die or be sawed off because they have died - but it is another thing altogether to saw them off when they are healthy.
Archaeologism at Vatican II
Despite the warnings of Pope Pius XII, there was a strong current of archaeologist thought among the liturgists of the day, many of whom saw medieval and Tridentine "accretions" as obstacles to engaging the faithful in the liturgy. It is uncertain how many prelates of the day were sympathetic to archaeologism, but it must of been considerable if Pius XII thought the subject merited mention and condemnation in Mediator Dei.
The leading exponent of the archaeological position at the Second Vatican Council was Abbot Basil C. Butler (1902-1986), Anglican convert, notable Scripture scholar and Abbot of the Benedictine congregation of Downside Abbey in England. Described paradoxically as a progressive committed to orthodoxy, Butler was a leading personality of the Council, and after Cardinal Bea, had the most influence on the document that would become Dei Verbum. In his 1966 autobiographical account of the Vatican II, Butler explains how he believed the Council needed to get "back to the source" of Revelation in order for the Church to present her message with the most meaning. In his words, an archaeologist presupposition became one of the "criterion" by which the Council's acts were to be judged:
"The pastoral aim, the instinct of a charity that goes beyond all boundaries, the sense of mission not so much to human nature or the abstract human species, but to human persons and the actually existing human family, demanded that our aggiornamento should be conceived of in depth. The consequent need to discriminate between what the Church must always be, what the Gospel forever is, and the contingent elements in which, at any given moment, the Church presents herself in history, was driving the Council to some criterion. And this drive took her gaze ever backwards, behind the counterrevolutionary Church, behind the Counter-Reformation, behind the medieval synthesis, back to the Church before the estrangement of East and West, to the Church before the confrontation with Greek culture and philosophy, to the primal source: to Christ in Palestine...Christ Himself is the fullness of divine revelation, and the content of the sacred tradition is just revelation, the Word of God made flesh." 
Several years later, after the promulgation of the Novus Ordo, the progressive revolution in full swing and Pius XII long in his grave, Butler was a little more candid and referred to the "accretions over the centuries" that the Council had thankfully swept away.  He expressed similar sentiments in his personal letters. In this revealing letter to his sister Mary Butler, he demonstrates how, far from viewing the developments of tradition as parts of the Church's identity, he sees them as the Church getting further and further from its primal roots, like a photocopy of a photocopy. These "remnants of past adaptations" have the effect of causing the Church to become "frozen." But let us look at Butler's words in context:
"[E]ver since the second century, the Church has been adapting herself to her age. In the second century she Europeanised herself having started as a Jewish thing. She learnt to think and express herself according to Greek philosophy. Then she Romanised herself (I might have added that, later still, she feudalised herself). She trails along with her remnants of these past adaptations. (I might have added that, since the Reformation she has frozen herself into the decadent medieval attitudes which she took up against the Reformers). For me renewal means not just a few changes in window dressing, but a radical return to her origins – not so that we become first century Palestinian Jews, but so that we may then ‘translate’ the fullness of the Gospel into terms relevant to our own age. (The Council can of course take only one or two tentative steps in the required direction; it will be for the Church to explore it further.)" 
Thus, the archaeologist theologian presumes a "radical return" to the attitudes of the first century will make the Gospel more "relevant" to the modern age. As to why the Christian attitudes of the first century are supposed to be more relevant than, say, the attitudes of the tenth of eighteenth centuries is not justified. It is presumed that older is better because it is nearer to the source. Tradition is like a photocopy of a photocopy that degrades in quality with each successive incarnation, until a "radical return" to our origins is necessary to break free from the "remnants of past adaptations."
Archaeologism, defined by Pius XII as an exaggerated antiquarianism, appears to be consonant with Catholic ecclesiology because it pays lip-service to the Church's history and tradition. In reality, however, it denies the Church's tradition because it proposes a drastically reductionist understanding of tradition as something limited to the initial traditio of the apostolic age. Unlike Cardinal Newman, who understood that Christianity as an idea could not but develop, and that these developments were essential expressions of Christian identity, archaeologism rejects many subsequent developments as "accretions" that hinder rather than express true Catholicity. Thus, the Church becomes an artifact that needs to be preserved and restored rather than a living reality that needs to be engaged in its historical fullness. The removal of many traditional practices and elements from Catholic life result in a positive weakening of the Church's ability to sanctify souls, because these practices were all instituted as a means to give glory to God and help the faithful in their Christian walk. The archaeologist presumes to strip down the Church to help the faithful; in reality, he strips down the resources of the faithful and wounds the Church.
"They are ever hunting for a fabulous primitive simplicity; we repose in Catholic fullness." -Cardinal John Henry Newman
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 Dei Verbum 8:1, CCC 98
 Pope Pius XII, Mediator Dei, 59
 ibid. 62
 ibid. 64
 Matt. 28:20; John 16:13
 Basil C. Butler, OSB, The Aggiornamento of Vatican II, 1966, "Faith Magazine, July 2013
 See Basil C. Butler, OSB, A Time to Speak (Southend on Sea, 1972)
 Basil C. Butler, Letter to Miss Mary Butler, 3 October 1964