48 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. For an example in point, Our predecessor of immortal memory, Pius IX, so argued when he proclaimed the Immaculate Conception of the Virgin Mary. Similarly during the discussion of a doubtful or controversial truth, the Church and the Holy Fathers have not failed to look to the age-old and age-honored sacred rites for enlightenment. Hence the well-known and venerable maxim, " Legem credendi lex statuat supplicandi " - let the rule for prayer determine the rule of belief. The sacred liturgy, consequently, does not decide or determine independently and of itself what is of Catholic faith. 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. But if one desires to differentiate and describe the relationship between faith and the sacred liturgy in absolute and general terms, it is perfectly correct to say, " Lex credendi legem statuat supplicandi " - let the rule of belief determine the rule of prayer. The same holds true for the other theological virtues also, " In . . . fide, spe, caritate continuato desiderio semper oramus " - we pray always, with constant yearning in faith, hope and charity.
Source: Mediator Dei (Vatican.va)