It’s an axiom in Anglican circles to use the phrase lex orandi, lex credendi when expressing doctrinal beliefs by appealing to liturgical texts. The question arises, however, whether this is a valid appeal and whether it actually happens. I will explore the following questions in this post:
What does this phrase mean and from where does it come?
Who are the theologians who have argued this?
What arguments have been advanced in support of this contention? Are they valid?
Is it true that Anglicans are reticent about definitions of belief but spend a good deal of time in developing liturgical texts?
1. What Does Lex orandi, Lex Credendi Mean? Where did it come from?
The phrase is actually a shorthand phrase for a longer phrase, ut legem credendi lex statuat supplicandi. Literally each word means:
“Ut” conjunction = “as, as being, so that”
“Legem” noun accusative masculine singular of lex = “law”
“Credendi” noun gerund genitive neuter singular of credo = “of belief”
“Lex” noun nominative masculine singular = “law”
“Statuat” verb present indicative 3rd singular of statuo = literally “causes to stand, set up” or “establishes, settles”
“Supplicandi” noun gerund genitive neuter singular from supplicare = “[of] kneeling, beseeching, entreating, praying to the gods.”
The shorthand phrase word by word means:
“Lex” noun accusative masculine singular = “law”
“Orandi” noun gerund genitive neuter singular from orare = of speaking , arguing, pleading [hence] praying”
“Lex” noun accusative masculine singular = “law”
“Credendi” noun gerund genitive neuter singular from credo = “of believing”
Therefore, the actual phrase means “so that the law of praying establishes the law of believing” and the famous shorthand is “law of praying, law of believing” or “law of prayer, law of belief.” Note that the verbal form shifts in the shorthand from supplicare to orare, possibly because the phrase lex supplicandi, lex credendi” isn’t quite as felicitous. Note also that the shorthand phrase does not contain a verb and the cases are in the nominative and genitive, so that grammatically the phrase could imply that either component, prayer or belief, leads to the other. However, the more complete phrase is clear in its direction: prayer establishes belief and not, at least grammatically, the other way around.
The context of [lex orandi, lex credendi] arises first in writings of Prosper of Aquitaine in the ca. 440s AD.The context of the phrase arises first in writings of Prosper of Aquitaine in the ca. 440s AD. was a disciple of St. Augustine and was writing over against the semi-Pelagians of his era, who argued that although salvation was God’s free gift, the first step was ordinarily taken on one’s own. Prosper argued against this as follows (emphases mine):
In addition, let us look at the sacred testimony of priestly intercessions which have been transmitted from the apostles and which are uniformly celebrated throughout the world and in every catholic church; so that the law of prayer may establish a law for belief. For when the presidents of the holy congregations perform their duties they plead the cause of the human race before the divine clemency and, joined by the sigh of the whole church, they beg and pray
that grace may be given to unbelievers;
that idolaters may be freed from the errors of their impiety;
that the Jews may have the veil removed from their hearts and that the light of truth may shine on them;
that heretics may recover through acceptance of the catholic faith….
that schismatics may receive afresh the spirit of charity;
that the lapsed may be granted the remedy of penitence;
and finally that the catechumens may be brought to the sacrament of regeneration and have the court of the heavenly mercy opened to them.
That these are not asked of the Lord lightly or uselessly is shown by the outcome. For God is pleased to draw many out of every kind of error, liberating them from the power of darkness and placing them in the kingdom of his beloved Son, turning them from vessels of wrath to vessels of mercy. This is felt to be so completely the work of God that the God who achieves it is always given thanks and praise for bringing such people to the light of truth.
Prosper was drawing a theological point – that we offer intercessions for people and therefore grace must come first, as we are praying for people whom we cannot even know ourselves – from liturgical texts, in this case the intercessory prayers of the Church, possibly at the daily Eucharist. Thus, the basic idea of liturgy as a source of theology. As Wainwright notes, this is quite usual during the Patristic periods. The 440s were not too far from 381 and the finalization of the Nicene-Constantinopolitan Creed at the Council of Constantinople, and Christianity was still very much working out what it understood about God, Christ, the Spirit, and salvation. Patristic authors didn’t have much formal theology from which to draw, so they did so primarily from Scripture, but also from the organic worship of the Church as it was being developed in multiple strands during this time.
 Latin definitions from Cassell’s Latin Dictionary (Hoboken: Wiley Publishing Inc., 1968) or Wheelock’s Latin 7th Edition (New York: HarperCollins 2011)
 So suggests The Rev. Dr. Thomas Williams, chair of the Dept. of Religious Studies at the University of South Florida.
 So claims Geoffrey Wainwright, Doxology, (New York: Oxford University Press 1980) 218
 Oxford Dictionary of the Christian Church (ODCC) 3rd edition p. 1481
 P. de Letter, SJ, translates the key phrase as a single sentence: “Let the rule of prayer lay down the rule of faith,” which implies an independence from the preceding sentence regarding looking at the prayers themselves. P. de Letter Prosper of Acquitaine: Defense of St. Augustine, (Westminster MD: The Newman Press 1963) 183.
 Wainwright, 225-26, his translation of the Latin original
 de Letter (234, footnote 42) suggests that these intercessions also look very similar to those of the Good Friday liturgy, as does Irwin (see below for citation).
 Wainwright 225-235 for an overview of Patristic sources of theology