The preoccupation among academics, for well over a century, with the 'problem' that the Roman Canon "lacks a theology of the Spirit" has, it seems to me, closed off some interesting lines of theological enquiry. For example: why is it that the Holy Spirit is absent from the NT narratives of the Last Supper and the Passion and Resurrection (except, just possibly, at John 19: 30)? He is central to all the accounts of the Lord's Baptism; and in the teaching of S Paul and S John about Christian Initiation (we receive the sphragis marking us with chrisma of the Spirit as a guarantee that we God's). In the classical Roman Rite, there is no shyness about involving the Holy Spirit in the formulae for Confirmation and for the Consecration of the Chrism. Might there be an interesting theological reason why, in the New Testament as in the immemorially ancient Roman Rite, each of the Persons of the Holy Trinity is involved in Baptism/Confirmation, while the Supper/Passion involves solely the sacrificial movement of the Son to His Father? The Classical Roman Rite might have unfolded fruitful truths to us, had its mouth not been stoppered by 'scholars' who preferred to be distracted by a different agenda.
My own, rather Patrimonial, view is that the liturgical forms we have inherited from so early in Christian history, particularly the Canon Romanus, have an authority similar to that of the other divine gifts which secured a normative status in those same early centuries, such as the Canon [i.e. official list] of books in the Bible; the three-fold Apostolic Ministry of Bishops, Presbyters, and Deacons; the Petrine Ministry. (Readers who wish to follow up my views on all this could look up auctoritas on the search engine.) These things are prescriptive in very much the same sort of way as S Prosper explained (see the first instalment of this post). So the Lex supplicandi should establish [statuat] the Lex credendi.
I am a trifle uneasy about the way in which Pius XII flirted (Mediator Dei, 1947) with the notion of reversing this formula so as to make the Law of Believing establish the Law of Praying, and of retaining both versions in, as it were, creative tension. This could easily relegate the Law of Praying to the humble status of a resource occasionally plundered by those drafting pontifical documents and needing proof-texts to bolster up a weak argument. This demotion of the Law of Praying seems indeed to be precisely what happened in the disordered years after the Council, when the levers of liturgical power in the Church fell into the hands of a tendenz determined to make their own presuppositions the dominant norm according to which liturgical texts would be judged and changed or even (as in the rite for the Consecration of bishops and by the provision of Alternative Eucharistic Prayers) dumped.
An expression of this desire to make the Lex credendi determine the Lex orandi (rather than the correct way round) can be found in the Decree of 1951 ordering a new Office for August 15: " ... congruum erat ut etiam Officium iis adornatum esset laudibus, quae Deiparae Virgini ob definitum corporeae Assumptionis dogma merito tribuendae erant".
As so often, when one really looks into matters, Pius XII turns out to be real progenitor of the "Post-Conciliar Reforms". Hannibal Bugnini forsaw this very clearly when he wrote in 1956 that the pope who had been, first, the Restorer of the Vigil and then the Restorer of Holy Week, would become the Supreme Restorer of the entire sacred Liturgy (totius sacrae Liturgiae Summus Instaurator).
We did not and do not need a Supreme Restorer of the entire Liturgy. We need a recovery of respect for the ancient liturgical texts so that they can form the beliefs of the worshipping community. The first step in true liturgical restoration should be, not so much the immediate prohibition of the entire Novus Ordo as the immediate outlawing of those Cuckoo's Eggs in the Roman Liturgy: the unRoman and orientalising Alternative Eucharistic Prayers.