Catechism of the Catholic Church

The Church's faith precedes the faith of the believer who is invited to adhere to it. When the Church celebrates the sacraments, she confesses the faith received from the apostles-whence the ancient saying: lex orandi, lex credendi (or: legem credendi lex statuat supplicandi, according to Prosper of Aquitaine [5th cent.]). The law of prayer is the law of faith: the Church believes as she prays. Liturgy is a constitutive element of the holy and living Tradition. (166, 1327, 78)