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 (AD 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.