Why does the liturgy always bear the brunt of the attacks launched by those intent on remaking the Catholic Church in the image of modernism? What is it about the sacred liturgy that poses such a threat to progressives, such that it suffers from the constant and unremitting tinkering of liberal 'reformers' intent on obliterating all vestiges of the Church's tradition? Of course, the Holy Sacrifice of the Mass is the most perfect act of worship and as such is especially hated by the devil, who rages against the Mass with a special hatred. This is obviously the supernatural motive behind progressive attacks. But there is also a very practical theological reason, and it is bound up with the Church's understanding of Tradition, Divine Revelation, and her own nature.
Theology is the study of Divine Revelation. Revelation comes from two sources: Sacred Scripture and Sacred Tradition. Scripture are those parts of revelation committed to writing at the inspiration of the Holy Spirit. Sacred Tradition, on the other hand, is those are those parts of revelation which were not committed to writing under the inspiration of the Holy Spirit. Note that whereas modern theological schools tend to define tradition as simply a "mode of transmission", Sacred Tradition must be considered in a composite sense, both in its object and in its act; or in other words, both what is handed on and how it is handed on. We must understand Tradition in terms of both content and mode, whereas moderns tend to focus on Tradition almost exclusively as a mode of transmission.
What exactly is the content of Tradition? When examining Tradition in terms of its content, theologians of the past distinguished two types of tradition - those which are constitutive and those which are inhesive. A tradition which is constitutive is something that "constitutes" a doctrine in and of itself; the tradition itself is the source if the doctrine. This means constitutive traditions are those doctrines and disciplines which are not found in the Scriptures. The Tradition itself "constitutes" the doctrine. Inhesive traditions, on the other hand, are those truths found in the Scriptures which are also found in Tradition. They are said to "inhere" in the Sacred Tradition, but are found principally in the Scriptures. In the case of inhesive tradition, Sacred Tradition is not the sole source but rather the support which testifies to the Scriptural teaching. It may mirror the Scriptural teaching, add to it, or may merely serve to clarify it.
Examples of inhesive traditions are plentiful. The sacrament of the Eucharist is clearly taught in the Gospels and in 1 Corinthians, and we also find many authoritative traditions touching on this sacrament. The Resurrection of our Lord Jesus is taught throughout the New Testament and also appears in the symbolon of the faith, that is, the creeds. An inhesive tradition appears in Scripture and Tradition.
But what are examples of constitutive tradition, those unwritten traditions which themselves constitute doctrines without reference to Scripture? Here we could cite the Assumption of the Blessed Virgin Mary, the Lenten Fast, clerical celibacy, the practice of granting Indulgences, and many other aspects of our Tradition which are not spelled out in the Scriptures. The most fundamental core of constitutive tradition, however, is the Church's liturgical heritage - her sacred rites surrounding the celebration of the sacraments. These are the centerpiece of constitutive tradition.
Traditional Catholic theology took it for granted that Traditions were both constitutive and inhesive; the fact appears without argument in Msgr. Geogre Agius' 1928 classic Tradition and the Church. Protestants, on the other hand, have traditionally denied the existence of any sort of authoritative constitutive tradition, since such an admission would necessitate the existence of an authority outside of Scripture.
With the rise of the new ecumenism and the school of the nouvelle theologie, however, Catholic theologians, too, began to debate whether there really is a constitutive tradition. As you may have inferred, this is closely bound up with a parallel modern debate concerning the sufficiency of the Sacred Scriptures. Some argue that the Scriptures are materially sufficient as a source of revelation; that is, every teaching of the Faith is found in the Scriptures in some sense, either explicitly or implicitly. Others (mainly progressives) argue that Scripture is formally sufficient; that is, every teaching of the Faith is found explicitly in the Scriptures. By the way, here is a decent article on the material vs. formal sufficiency debate.
The debate over material vs. formal sufficiency ties in with the debate over constitutive vs. inhesive. Obviously, if Scripture is materially sufficient, then there remains room for a real constitutive tradition, but not if it is formally sufficient. The same progressive theologians who view tradition as merely a mode of transmission and deny the existence of a constitutive tradition simultaneously argue that the Scriptures are formally sufficient, as this undermines the existence of constitutive tradition from another angle.
Why do progressives array themselves against constitutive tradition? Precisely because constitutive tradition contains all those elements of Catholicism most offensive to liberals. But these constitutive traditions are not isolated little units sitting out there on their own waiting for liberals to chuck them; they are in fact all bound up and integrated into the Church's perennial liturgy, which forms the centerpiece and bulwark of the Church's constitutive tradition. There is no way to get at the constitutive unless the liturgy is overthrown.
Hence the Sacred Liturgy as the focal point of liberal attacks. The most 'offensive' aspects of our Faith can only be isolated and diminished once liturgy is out of the way, as the liturgy "encases" them like an insect encased and preserved in amber. The new ecumenism, pioneered by such progressives as Dom Lamber Beauduin and Fr. Yves Congar, had adopted the principle that those aspects of Catholicism which were most offensive to non-Catholic Christians ought to be minimized in order to remove stumbling blocks to Christian unity (see here). Therefore, as the most 'offensive' aspects of Catholicism from a non-Catholic viewpoint are encapsulated in the liturgy, the liturgy must be remade.
This explains why liturgy is the central opponent of progressives. Liturgy is the embodiment of constitutive tradition. Constitutive tradition is those things which cannot be found explicitly in Scriptures and hence are most offensive to Protestants. Therefore they - and the liturgy which protects and embodies them - must be sacrificed at the altar of ecumenism.
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