heresy, in religion, especially in Christianity, beliefs or views held by a member of a church that contradict its orthodoxy, or core doctrines. It is distinguished from apostasy, which is a complete abandonment of faith that makes the apostate a deserter, or former member. Heresy is also distinguished from schism, which is a splitting of or from the church brought about by disputes over hierarchy or discipline, rather than over matters of doctrine. The heretic considers himself or herself not only a church member but, in a doctrinal controversy, the true believer; indeed, some persons originally labeled heretical were rehabilitated after once abhorred views become accepted.

The battle for doctrinal control of Christianity began with the declarations of St. Paul in the New Testament. In the religion's first three centuries, numerous sects, many arising from Gnosticism, were in conflict. The first Council of Nicaea (A.D. 325), which addressed the challenge of Arianism, was among convocations at which a Christian orthodoxy was established.

Excommunication was the usual method of dealing with heretical individuals or small groups. The medieval church undertook military action (as against the Albigenses, in 1208) and extensive legal and punitive campaigns (such as the Inquisition) in striving to suppress large-scale heresy. The Protestant Reformation created new churches that at first campaigned against heresy from their own doctrinal bases; over time, however, the Roman Catholic church has remained the only Christian body that has continued with any frequency, on the basis of canon law, to prosecute heretics.

See also blasphemy.

Doctrine rejected as false by religious authorities. In Christianity, the orthodox theology of the church is thought to be based on divine revelation, and heretics are viewed as perversely rejecting the guidance of the church. Numerous Christian heresies appeared from the 2nd century onwards. Early heresies included Arianism, the Monophysite heresy, Pelagianism, and Donatism. Some heresies, such as Montanism, expressed faith in a new prophet who added to the body of Christian revelation. Some types of Gnosticism were heretical branches of Christianity. The major means of combating heretics in the early church was excommunication. In the 12th–13th century, the Inquisition was established to combat heresy, and heretics who refused to recant were often executed. In the 16th century the Protestant Reformation brought an end to the doctrinal unity of Western Christendom, and the concept of heresy became less important in the various Christian churches, though it continues to exist. The concept of heresy also exists in Judaism, Buddhism, Hinduism, and Islam.

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(5th–6th century AD) Doctrine that emphasized the single nature (the term means literally “of one nature”) of Christ, as a wholly divine being rather than part-divine and part-human. Monophysitism began to appear in the 5th century; though condemned as a heresy at the Council of Chalcedon (451), it was tolerated by such Byzantine leaders as Justin II, Theodora, and Zeno, resulting in a full-fledged schism between East and West. Several Monophysite churches, including the Coptic Orthodox Church, were founded in the 6th century.

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