Its simplistic applicability as a self-standing principle independent of hope and charity was bluntly denied by Pope Pius XII, who positioned liturgy as providing theological evidence not authority.
The original maxim is found in 5th Century writer Prosper of Aquitaine's eighth book on the authority of the past bishops of the Apostolic See concerning the grace of God and free will: "ut legem credendi lex statuat supplicandi."
"However, in the encyclical Mediator Dei, Pope Pius XII elucidates but strongly limits this principle and addresses errors that can arise from a misunderstanding of it.
[...]We refer to the error and fallacious reasoning of those who have claimed that the sacred liturgy is a kind of proving ground for the truths to be held of faith, meaning by this that the Church is obliged to declare such a doctrine sound when it is found to have produced fruits of piety and sanctity through the sacred rites of the liturgy, and to reject it otherwise.
For this reason, whenever there was question of defining a truth revealed by God, the Sovereign Pontiff and the Councils in their recourse to the "theological sources," as they are called, have not seldom drawn many an argument from this sacred science of the liturgy.[...
More properly, since the liturgy is also a profession of eternal truths, and subject, as such, to the supreme teaching authority of the Church, it can supply proofs and testimony, quite clearly, of no little value, towards the determination of a particular point of Christian doctrine.
Digging deep into the sources, the theologian and the liturgist aim simply to penetrate the profundity of the mystery of the faith as it has shown itself in the concrete life of the Church all through her history.
[8] In the interpretation of so-called liturgical theology radical theologians, espoused for example by some Traditionalist Catholics, liturgy determines doctrine rather than the inverse.
[9] For them, Prosper of Aquitaine's formulation establishes the credence of certain Christian doctrines by placing their source in the Church's authentic liturgical rites, thus describing the liturgy itself as a deposit of extra-Biblical Christian revelation (part of a body of extra-Biblical beliefs known more collectively as Apostolic tradition), to which, in addition to Scripture, those who wished to know true doctrine could also refer.
Councils and creeds recognized as authoritative are interpreted only as defining and more fully explicating the orthodox faith handed to the apostles, without adding to it.