Lex orandi, lex credendi

Lex orandi, lex credendi (Latin: "the law of what is prayed [is] the law of what is believed"), sometimes expanded as Lex orandi, lex credendi, lex vivendi (Latin: "the law of what is prayed [is] what is believed [is] the law of what is lived"), is a motto in Christian tradition, which means that prayer and belief are integral to each other and that liturgy is not distinct from theology. It refers to the relationship between worship and belief, for example that people's prayers shape their faith. Its rude applicability as a self-standing principle independent of hope and charity was denied by Pope Pius XII, who positioned liturgy as providing theological evidence not sole authority, although having undeniable divine elements.

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."

Let us consider the sacraments of priestly prayers, which having been handed down by the apostles are celebrated uniformly throughout the whole world and in every Catholic Church so that the law of praying might establish the law of believing" (emph. added)

— Prosper of Aquitaine

"Supplicandi" and “Credendi" are gerunds which are oblique cases of the infinitive, and so can be translated into English as "of praying/believing," or just "of prayer/belief," respectively.

The principle is considered very important in Catholic theology. The Catholic theologian Father Gregory Hesse states:

"The oldest liturgical rule is 'lex orandi, lex credendi', i.e. 'the law of what has to be prayed constitutes the law of what has to be believed (...) Liturgy is a sign of a specific grace received ex operæ operatio"

— A Summary of Father Hesse talks on the documents of Vatican II