This ancient Latin axiom is quoted so often, I thought a little explanation of it on our web site would be helpful. A paraphrase of a longer patristic expression, the phrase means, "the law of praying is the law of believing."
The Father of the Church who gave us the axiom is St. Prosper of Aquitaine. He coined it in his controversy with the semi-Pelagians, who held that God's grace was necessary neither for one's first movement towards conversion nor for final perseverance.
According to Prosper of Aquitaine, legem credendi lex statuat supplicandi, which is to say, 'the law of prayer determines the law of belief' (Prosper used the equivalent term lex supplicandi in place of lex orandi ). Prosper treats the church's prayer as an authoritative source for theology in arguing that salvation must come entirely at God's initiative since in the liturgy the church prayed for the conversion of infidels, Jews, heretics, schismatics and the lapsed who would not seek the true faith on their own. (Charles R. Hohenstein, “‘Lex Orandi, Lex Credendi’: Cautionary Notes “. Cf. Prosper of Aquitaine, De vocatione omnium gentium, 1, 12: PL 51, 664C.)
The same phrase turns up in an official document of the Holy See, Indiculus, which was a compilation of all the authoritative statements of the popes on the subject of grace. It is believed that this document was edited by St. Prosper himself, as he was Pope St. Celestine's secretary at the time. Here is the relevant passage, as contained in Denzinger's:
Let us be mindful also of the sacraments of priestly public prayer, which handed down by the Apostles are uniformly celebrated in the whole world and in every Catholic Church, in order that the law of supplication may support the law of believing.
For when the leaders of the holy nations perform the office of ambassador entrusted to them, they plead the cause of the human race before the divine Clemency, and while the whole Church laments with them, they ask and pray that the faith may be granted to infidels; that idolaters may be delivered from the errors of their impiety; that the veil of their hearts may be removed and the light of truth be visible to the Jews; that heretics may come to their senses through a comprehension of the Catholic faith; that schismatics may receive the spirit of renewed charity ; that the remedy of repentance may be bestowed upon the lapsed; that finally after the catechumens have been led to the sacraments of regeneration, the royal court of heavenly mercy may be opened to them. (Indiculus, chapter 8; Denz., n. 246 [old edition, n. 139], emphasis ours.)
The editors of Denzinger's inserted a footnote stating that the entirety of chapter eight of this decree agrees with St. Prosper's De vocatione omnium gentium, where the argument first appeared. They also refer the reader to the ancient Solemn Prayers we described above as having been excised from the new Missal. Doubtless, St. Prosper had heard these prayers on Good Friday, as liturgical historians date them back to the earliest persecutions. He probably had them in mind when he wrote this passage.
This highlights the grave importance of tradition in the Holy Sacrifice of the Mass and all the Church's liturgy. It also shows us that the liturgy itself is a powerful source of Christian truth.
When we Latin Catholics of the West return to our liturgical traditions and show that we take this axiom seriously, the Eastern Orthodox — for whom tradition, liturgy, and the rule of faith are virtually synonomous — will take Catholic unity under the Pope more seriously.