Heresy, any religious doctrine opposed to the dogma of a particular Church, especially a doctrine held by a person professing faith in the teachings of that Church. The term originally meant a belief that one arrived at by oneself (Greek, hairesis, “choosing for oneself”) and is used to denote sectarianism in the Acts of the Apostles and in the Epistles of St Paul. In later Christian writings, the term is used in the opprobrious sense of a belief held in opposition to the teaching of the Church.

With the establishment of Christianity in the Roman Empire, heresy came to be considered a crime against the state, punishable by civil law. Heresy was also generally outlawed in countries with an established or state-supported Church. After the Reformation, however, the principles of private interpretation of the Scriptures and denial of ecclesiastical authority in all matters of belief were eventually adopted in Protestant countries, and during the 19th and 20th centuries Roman Catholic countries also adopted the principle of religious toleration.