It's become something of a commonplace, this shorthand version of an ancient Latin aphorism: Lex orandi, lex credendi. Simply translated, in this form it says: "The law of praying [is] the law of believing."
But what does this mean?
The phrase has been bandied about quite a bit since the 1960s, also among Lutherans here and there, as among Roman Catholics and pretty much anyone else interested in liturgical practice over the past half a century. From time to time it seems to be popping up all over the place, and then it recedes more quietly into the background again. Having long since become something of a cliché, the phrase rarely prompts or propels a discussion, but it's tossed into conversation along the way to mark a point that most everyone acknowledges to one degree or another. The fact that it's usually cited in Latin lends it a certain mystique that defies objection or argument.
The way that "lex orandi, lex credendi" is usually used and understood is simply a truism. That is, the way (and the what) that one prays is intimately and reciprocally related to the way (and the what) that one believes. "Prayer" here refers broadly to the full scope of liturgical practices. In short, doctrine informs practice, but practice in turn shapes doctrine. What the church does, and how she does it, not only confesses but in time also determines what she believes and teaches. The practice of prayer is a way of teaching; indeed, it is a primary means of catechesis, which forms thinking and believing.
This observation is interesting, and it bears consideration. It belies the misleading but all too common notion that one can separate the "style" of worship from the "substance" of the faith. If a church's practice is not faithfully in harmony with sound doctrine, then its teaching, believing and confessing will follow its practice into heterodoxy. Indeed, a false practice is already a false teaching and a false confession in its own right, a pseudo creed that catechizes its participants, whether witting or unwitting, in its false faith and false life.
Conversely, faithful practice — that is to say, a liturgical context in harmony with the right preaching of the Gospel and the right administration of the Sacraments — powerfully catechizes its participants, both young and old, pastors and laity, elders and neophytes, infants and the whole glad throng, in the true faith and life of Christ our Lord.
So we are rightly concerned with "prayer," broadly speaking, as a teaching and confessing of belief. Right praying serves, supports and substantiates right believing, in much the same way that heterodox praying is both indicative and precipitant of heterodox believing. That's basically the affirmation and the warning implicit in "lex orandi, lex credendi," as it is typically summoned to duty.
The aphorism is traced especially to St. Prosper of Aquitaine (fifth century). I'm working from memory here, but here's the gist of it. Prosper was an Augustinian theologian, as we might say, defending the necessity of divine grace for conversion and salvation. It was in that context that he wrote, "ut legem credendi lex statuat supplicandi." Here, "supplicandi" is synonymous with "orandi." No problem there. The original is not quite so short and sweet as the shorthand, "lex orandi, lex credendi," but it basically says that the "law of praying" establishes the "law of believing."
What, then, is the "law of praying"? Prosper actually had a quite particular "law" or "rule" in mind, namely, the admonition of St. Paul in his first epistle to St. Timothy: "First of all, I urge that entreaties and prayers, petitions and thanksgivings, be made on behalf of all men," because "God our Savior desires all men to be saved and to come to the knowledge of the truth" (1 Timothy 2:1-4). Prosper could then point to the actual practice of bishops all over the world, who interceded for the conversion and salvation of Jews, pagans, heretics and so forth, in the prayers of the Church; which practice was determined by this "law of praying," commanded by the holy Apostle in the name of the Lord. As it is necessary to pray for the conversion and salvation of "all men," it is established that the grace of God is necessary for anyone to be saved.
In other words, it is not simply the Church's tradition or practice of prayer, but the Church's obedience in praying according to the divinely-given "law of praying," which attests to and confirms her faith. The Church prays as she has been commanded to pray (by the One who promises to hear), and that lex supplicandi defines the Church's creed, her teaching and confession: the lex credendi. For St. Prosper of Aquitaine, in the case at hand, the Lord's command to pray "for all men" establishes the Church's confession of divine grace.
Lutherans have argued similarly with respect to infant Baptism. As the Lord Jesus commands His Apostles to make disciples of "all nations" by baptizing and teaching them, it follows that all people, including infants, should be baptized (and taught). Accordingly, Luther could also point to the Church's practice in this regard as one of his arguments against the anabaptists; for the Church had been doing what the Lord commanded, and her practice was honored by Him in the regeneration and faithful witness of those who were baptized as infants.
Luther likewise argues from the fifth petition, "forgive us our trespasses," that forgiveness is a gracious gift of God, which even Christians require on a daily basis throughout their life on earth. The Lord's command that we should pray in this way, and His promise to hear us, is a teaching and confession of our need and God's gift of forgiveness. By the same token, the fact that we are taught to pray at all demonstrates that we must look to Him in faith and rely upon Him for every good thing, for all that we need in both body and soul. In this way, too, the "law of praying" serves, supports and substantiates the "law of believing." It belongs not only to the Lord's teaching of the Our Father (St. Matt. 6:9-13; St. Luke 11:1-4), but also to the other fundamental lex orandi of the New Testament: "Pray without ceasing" (1 Thess. 5:17; also St. Luke 18:1).
This original definitiion of the "lex orandi" is instructive. It begins with the understanding that the Church's practice of prayer is not a self-authenticating enterprise. Faithful praying takes its stand upon, and take its cues from, the Word and promises of God. Which is not to say that every rubric, rite and ceremony must have an explicit command. That we should pray "for all men" and "without ceasing" requires a good ordering of life and practice, which need not be the same at all times and in all places. As preaching properly divides the Word of truth, proclaiming the Law and the Gospel in many and various ways, so does faithful praying proceed according to various patterns of sound words. Yet, faithful preaching and praying alike derive their authority from the Holy Scriptures, from the doctrine of the blessed Apostles. Nothing dare undermine, contradict, or compete with that regula fidei (the rule of faith). But the Church, in the freedom of the Gospel, regulates her practice according to and in harmony with the Lord's "lex orandi," in order that her faith in Him may be rightly and clearly confessed. In this way, the Church's liturgical practice establishes an orthodox "lex credendi."