Heresy is any belief or theory that is strongly at variance with established beliefs or customs, in particular the accepted beliefs of a church or religious organization. The term is usually used in reference to violations of important religious teachings, but is also used of views strongly opposed to any generally accepted ideas. A heretic is a proponent of heresy.The term is used particularly in reference to Christianity, Judaism, and Islam. In certain historical Christian, Muslim, and Jewish cultures, among others, espousing ideas deemed heretical has been (and in some cases still is) 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.

Johann Sylvan (died 23 December 1572) was a Reformed German theologian who was executed for his heretical Antitrinitarian beliefs.