Creeds (Latin credo,”I believe”), authoritative summaries of the principal articles of faith of various churches or bodies of believers. As religions develop, doctrines that were originally simple are subject to elaboration and interpretation that cause differences of opinion. Detailed creeds become necessary to clarify the differences between the tenets of schismatic branches and to serve as formulations of belief when liturgical usage—for example, the administration of baptism—requires a profession of faith.

In the Christian church, the Apostles' Creed was the earliest summation of doctrine; it has been used with only minor variations since the 2nd century. In addition to the Apostles' Creed, the Nicene Creed and the Athanasian Creed are in common use in the Roman Catholic liturgy. In the Orthodox church, the only creed formally adopted was the Nicene Creed, without the insertion of filioque in connection with the procession of the Holy Spirit.

With the Reformation, the establishment of the various Protestant churches necessitated the formulation of new creeds, which, because of the many differences in theology and doctrine, were much longer than the creeds of the ancient church. The Augsburg Confession is accepted by Lutherans throughout the world, as is the Smaller Catechism of Martin Luther. The Formula of Concord, accepted by most early Lutherans, now finds more limited acceptance. The doctrines of the Church of England are summarized in the Thirty-nine Articles, and those of the Presbyterians, in the Westminster Confession. Most Reformed churches of Europe subscribe to the Helvetica Posterior, or Second Helvetic Confession, of the Swiss reformer Heinrich Bullinger, and most Calvinists accept the Heidelberg Catechism.