Heresy is any belief or theory that is strongly at variance with established beliefs or customs, particularly the accepted beliefs or religious law of a religious organization. A heretic is a proponent of heresy.
Heresy in Christianity, Judaism, and Islam has at times been met with censure ranging from excommunication to the death penalty.
Heresy is distinct from apostasy, which is the explicit renunciation of one's religion, principles or cause; and from blasphemy, which is an impious utterance or action concerning God or sacred things. Heresiology is the study of heresy.
Derived from Ancient Greek haíresis (αἵρεσις), the English heresy originally meant "choice" or "thing chosen". However, it came to mean the "party, or school, of a man's choice", and also referred to that process whereby a young person would examine various philosophies to determine how to live.
The word heresy is usually used within a Christian, Jewish, or Islamic context, and implies slightly different meanings in each. The founder or leader of a heretical movement is called a heresiarch, while individuals who espouse heresy or commit heresy are known as heretics.
Heresy in Christianity
According to Titus 3:10 a divisive person should be warned twice before separating from him. The Greek for the phrase "divisive person" became a technical term in the early Church for a type of "heretic" who promoted dissension. In contrast, correct teaching is called sound not only because it builds up the faith, but because it protects it against the corrupting influence of false teachers.
Tertullian (AD 155–240) implied that it was the Jews who most inspired heresy in Christianity: "From the Jew the heretic has accepted guidance in this discussion [that Jesus was not the Christ]."
The use of the word heresy was given wide currency by Irenaeus in his 2nd-century tract Contra Haereses (Against Heresies) to describe and discredit his opponents during the early centuries of the Christian community.