heresy

Heresy is the holding of beliefs or opinions that deviate from the established orthodox doctrine of a religious community. The specific content of orthodoxy varies across traditions and eras, making the boundary between accepted belief and heresy a social and doctrinal construct rather than a fixed fact.

The word derives from the Greek haireisis or haeresis meaning "a choice" or "a sect," through Latin haeresis and Old French hérésie. It developed in Christian polemics to label beliefs judged contrary to the faith of the church.

In Christian history, heresy referred to doctrines deemed incompatible with the church's official faith. Debates over Christology, the nature of salvation, and church authority led to condemnations at ecumenical councils such as Nicea, Constantinople, and later gatherings. Heresy was followed by disciplinary measures such as excommunication, and in some periods by civil penalties or inquisitorial proceedings. Notable medieval examples include Arianism, Gnosticism, Donatism, and Pelagianism; later, movements associated with the Reformation generated further orthodoxy disputes.

Today, the term is used more loosely and often within particular communities; outside theology, it can describe unorthodox opinions in philosophy, science, or politics. In Islam, parallel concerns exist about deviations, expressed through terms such as bid'ah or kufr rather than the Christian concept of heresy. In many countries, formal punishments for dissent have largely disappeared, and freedom of belief is protected, although disputes over doctrine and authority persist. The study of heresies, or heresiology, remains an area of historical and religious scholarship.