It is a favorite expression of many, Lex Orandi Lex Credendi. Literally, it translates "The law of prayer is the law of belief." It means, in practical terms, that as people pray, so they believe. At its best it supports the idea that the Book of Common Prayer is a Formulary, and that by liturgical prayer the Church passes on its teaching through practice. This is why our service of Holy Communion is one of the best evangelistic and catechetical tools we have. It teaches the whole Gospel every time it is celebrated. Every part of it, the Creed, the Canon of Consecration, etc., is at one time prayer, worship, evangelism and catechesis. The offices of Daily Morning and Evening Prayer also instruct as they are practiced.
Nonetheless, it is not wise for clergy to depend so heavily on this concept that they fail to teach diligently. The Canon Law of the ACC requires, in full consistency with the practice of the Church from earliest times, that at the very least every Sunday the priest shall preach, or cause to be preached, a sermon in the church. This is the minimum standard for carrying out his work as a minister of God's word, not the maximum. A sermon is not a seven minute homily carried out as a necessary evil; it is the preaching of God's word with preparation and prayer, carried out with diligence, produced by true work of the most responsible kind. Yes, the truth is well expressed in the liturgy, but somehow the human mind cannot comprehend it without teaching and preaching.
Repetition has both an advantage and a disadvantage. The advantage is that it enters the long term memory and stays there. Every priest who visits the elderly in nursing homes to bring Communion to them, when dealing with those who are barely able to speak and whose eyes can no longer read, sees that people raised on the Book of Common Prayer are joining in, praying from memory, moving their lips. It works.
But, the disadvantage is that people who recite the words can shut out the meaning. One elderly woman, greeting a rector after a service in the Episcopal Church in 1980, unhappy with their 1979 book, complained: "If Jesus could hear that new book, he would roll over in his grave." She had said the words, "On the third day he rose again" thousands of times; but, somehow, she had ceased to hear those words. Lex Orandi is not enough, by itself, to produce Lex Credendi; the teaching and preaching ministry remains a chief responsibility of all priests.
Finally, at its worst, Lex Orandi Lex Credendi can be misused in theological debate. Individuals may insist that they can raise pious opinion to the level of dogma, or perhaps even argue erroneous opinion, by treating all approved or allowed prayers as if they were equal to the Canon of Scripture; though the latter may involve twisting the meaning of prayers or failing to think them through. This approach does not work, however, on orthodox theologians.
For me, the slogan is more accurate if we reverse it: Lex Credendi Lex Orandi: As I believe, so I pray. After years of reading and studying the word of God, by which I mean the holy Scriptures, I am able to pray only according to my convictions. Eventually, for every student of theology, this must become the case. And, this too is a strength of our Book of Common Prayer tradition. Those who really know the Scriptures have no qualms about saying "amen" to the prayers in that book. C.S. Lewis pointed out that this is an advantage to having a proper liturgy instead of public prayers composed by a minister or improvised: No one fears to say "amen." Our Book of Common Prayer is so good and sound, that the more we know the Bible the more easily and readily we use our liturgy, and the more readily we say "amen" to it.