Pius XI on liturgy

Since the Church has received from Christ her Founder the office of safeguarding the sanctity of divine worship, it is certainly incumbent upon her, while leaving intact the substance of the Sacrifice and the sacraments, to prescribe ceremonies, rites, formulae, prayers and chant for the proper regulation of that august public ministry, whose special name is “Liturgy”, as being the eminently sacred action. For the liturgy is indeed a sacred thing, since by it we are raised to God and united to Him, thereby professing our faith and our deep obligation to Him for the benefits we have received and the help of which we stand in constant need. There is thus a close connection between dogma and the sacred liturgy, and between Christian worship and the sanctification of the faithful. Hence Pope Celestine I saw the standard of faith expressed in the sacred formulae of the liturgy. “The rule of our faith,” he says, “is indicated by the law of our worship. When those who are set over the Christian people fulfill the function committed to them, they plead the cause of the human race in the sight of God’s clemency, and pray and supplicate in conjunction with the whole Church.” (1 Epist. ad episcopos Galliarum, Patrol. Lat., L, 535.)

Apostolic constitution Divini cultus by Pius XI.