Lex orandi, lex credendi
Lex orandi, lex credendi (Latin loosely translatable as "the law of prayer is the law of belief") refers to the relationship between worship and belief, and is an ancient Christian principle which provided a measure for developing the ancient Christian creeds, the canon of scripture and other doctrinal matters based on the prayer texts of the Church, that is, the Church's liturgy. In the Early Church there was liturgical tradition before there was a common creed and before there was an officially sanctioned biblical canon. These liturgical traditions provided the theological framework for establishing the creeds and canon.
An early account of the maxim is found in 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, "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".
Roman Catholicism 
The principle is considered very important in Catholic theology. The Catechism of the Catholic Church states: "The Church's faith precedes the faith of the believer who is invited to adhere to it. When the Church celebrates the sacraments, she confesses the faith received from the apostles - whence the ancient saying: lex orandi, lex credendi (or: legem credendi lex statuat supplicandi, according to Prosper of Aquitaine). The law of prayer is the law of faith: the Church believes as she prays. Liturgy is a constitutive element of the holy and living Tradition."
At a symposium held in connection with the publication of a set of reproductions of the first editions of the Tridentine liturgical texts, including the Roman Missal and the Roman Breviary, Archbishop Piero Marini, former Master of Pontifical Liturgical Celebrations, presented a paper entitled "Returning to the Sources", in which he said: "It is above all in the Liturgy that renewal cannot do without a sincere and profound return to the sources: sources of that which is celebrated and sources of that which is believed (lex orandi, lex credendi). Digging deep into the sources, the theologian and the liturgist aim simply to penetrate the profundity of the mystery of the faith as it has shown itself in the concrete life of the Church all through her history."
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Lex orandi, lex credendi is a fundamental character of Anglicanism. Its importance is due primarily to the fact that the Scriptures are the primary source of authority for Anglican theology (See Article VI of the Thirty-nine Articles. Although other traditions which take their name from their founding theologian (e.g., Calvinism, Lutheranism, Mennonite, or Zwinglianism) the Anglican Reformation is no less appreciative to the father of the English Reformation, Archbishop Thomas Cranmer. The position of the English Reformation is that the church is subject to Scripture, whereas Anglo-Catholicism affirms that Tradition is equal to Scripture, which implies that the institutional church possesses equal control over the content of orthodox Christian doctrine. This difference is the great divide between the Protestant and English Reformation and the Roman Catholic Church and the Anglo-Catholic sympathizers with Rome. The via media is an attempt to revise the English Reformation in a more Roman Catholic direction. Other Anglicans would disagree that Scripture is the primary source of authority and insist with Richard Hooker (qv) that Scripture, Tradition and Reason must be held in tension as of equal import and authority. Indeed the phrase Lex orandi lex credendi states that it is in in our worship that we express our beliefs and that, in itself, is a form of authority.
Instead, Anglicans have what are called the Anglican Formularies to guide Anglican theology and practice. The Anglican Formularies are the Thirty-nine Articles of Religion, the 1662 Book of Common Prayer, and the Ordinal. The Archbishop of Canterbury, Thomas Cranmer, principal author of the prototypical 1549 BCP and the more Reformed 1552 BCP, could be said to be the first Anglican theologian. His theology is expressed in the selection, arrangement, and composition of prayers and exhortations, the selection and arrangement of daily scripture readings (the lectionary), and in the stipulation of the rubrics for permissible liturgical action and any variations in the prayers and exhortations - though, of course, his selections and arrangements were based on pre-existing continental Reformed theology. Gregory Dix, the Anglo-Catholic theologian has well said that Thomas Cranmer was a liturgical genius who helped to make the doctrine of justification by faith alone part of the common faith of England through the later 1662 Book of Common Prayer, which was faithful to the 1552 Book of Common Prayer. Elizabeth I, being Protestant, wanted to maintain the Protestant faith in England, though she did not allow the Puritans to regain control. It should be noted that "justification through faith alone" is not a phrase much used in "broad church" Anglicanism. Similarly the term "Protestant" sits uncomfortably with many Anglicans. Indeed Anglicanism is better described as "Catholic and Reformed".
Given its locus in the worship of the Church, Anglican theology tends to be Augustinian and Reformed and embodies a strongly evangelistic liturgy, according to Samuel Leuenberger's book, Archbishop Thomas Cranmer's Immortal Bequest. (See: Churchman 106/1 1992: Archbishop Cranmer’s Immortal Bequest: The Book of Common Prayer of the Church of England: An Evangelistic Liturgy) The genius of Cranmer was in employing the principle of lex orandi, lex credendi to teach the English congregations the Reformed doctrines of grace and the sine qua non of the Gospel, justification by faith alone.
Eastern Orthodoxy's Patriarch Bartholomew I of Constantinople quoted this phrase in Latin on the occasion of the visit of Pope Benedict XVI, drawing from the phrase the lesson that, "in liturgy, we are reminded of the need to reach unity in faith as well as in prayer." Rather than regarding Tradition as something beneath Scripture or parallel to Scripture, Orthodox Christians consider Scripture the culmination and supreme expression of the church's divinely communicated Tradition. Councils and creeds recognized as authoritative are interpreted only as defining and more fully explicating the orthodox faith handed to the apostles, without adding to it.
- Patrologia Latina 51:209-210. ...obsecrationum quoque sacerdotalium sacramenta respiciamus, quae ab apostolis tradita, in toto mundo atque in omni catholica Ecclesia uniformiter celebrantur, ut legem credendi lex statuat supplicandi
- Catechism of the Catholic Church, 1124
- Monumenta Liturgica Concilii Tridentini
- http://www.va/news_services/liturgy/2006/documents/ns_lit_doc_20060323_ritorno-fonti_en.html Returning to the Sources
- Homily on the Feast of Saint Andrew on 30 November 2006
- William R. Crockett, Eucharist: Symbol of Transformation. New York: Pueblo, 1989.
- W. Taylor Stevenson, “Lex Orandi—Lex Credendi.” In The Study of Anglicanism, ed. by Stephen Sykes and John Booty. London: SPCK, 1988, pp. 174-88.
- William J. Wolf, “Anglicanism and Its Spirit.” In The Spirit of Anglicanism: Hooker, Maurice, Temple, ed. by William J. Wolf. Wilton, CT: Morehouse-Barlow, 1979.
- Paul de Clerck, “Lex orandi, lex credendi”: The Original Sense and Historical Avatars of an Equivocal Adage. In Studia Liturgica 24, 1994, 178-200