Prosper of Aquitaine

"Lex orandi lex credendi" ("The rule of prayer is the rule of belief"). For Catholic Christians, "lex orandi lex credendi" celebrates living Tradition in the form of the liturgy, breathing life into God's self-communication that triumphs over time and space to reach us in our own era. Church historians credit Prosper of Aquitaine with coining this important axiom in the fifth century while he was a student of St. Augustine of Hippo. Prosper's adage captures the essential Catholic belief that the liturgy--the Church's collective prayers, our sacraments, and our public rituals--expresses our adoration, gratitude, and supplication to God the Father, through Jesus Christ, in unity with the Holy Spirit. In the liturgy the Church reaches into the beating heart of Jesus Christ and his Church to emerge clothed in the will of our creator communicated to us in prayer, sacraments, and symbols. 
Sources: Richard P. McBrien. Catholicism. San Francisco: HarperCollins, 1994