Heresy is an introduced change to some system of belief, especially a religion, that conflicts with the previously established canon of that belief.

The study of heresy is heresiology. The founder or leader of a heretical movement is called a heresiarch. One who espouses heresy is called a heretic.

Etymology

The word "heresy" comes from the Greek , hairesis (from , haireomai, "choose"), which means either a choice of beliefs or a faction of believers, or a school of thought. It was given wide currency by Irenaeus in his tract Contra Haereses (Against Heresies) to describe and discredit his opponents in the early Christian Church. He described his own position as orthodox (from ortho- "straight" + doxa "belief") and his position eventually evolved into the position of the early Christian Church.Fact: date=January 2009

Used in this way, the term "heresy" has no purely objective meaning: the category exists only from the point of view of speakers within a group that has previously agreed about what counts as "orthodox". Any nonconformist view within any field may be perceived as "heretical" by others within that field who are convinced that their view is "orthodox"; in the sciences this extension is made tongue-in-cheek.

Heretics usually do not define their own beliefs as heretical. Heresy is a value judgment and the expression of a view from within an established belief system. For instance, Roman Catholics held Protestantism as a heresy while some non-Catholics considered Catholicism the "Great Apostasy."

For a heresy to exist there must be an authoritative system of dogma designated as orthodox, such as those proposed by Catholicism. The term orthodox is used in Eastern Orthodoxy, some Protestant churches, in Islam, some Jewish denominations, and to a lesser extent in other religions. Variance from orthodox Marxism-Leninism is described as "right" or "left deviationism." The Cult of Scientology uses the term "squirreling" to refer to unauthorized alterations of its teachings or methods.

Christianity

main: Christian heresy

In Christianity, heresy is a "theological or religious opinion or doctrine maintained in opposition, or held to be contrary, to the Roman Catholic or Orthodox doctrine of the Christian Church, or, by extension, to that of any church, creed, or religious system, considered as orthodox. By extension, heresy is an opinion or doctrine in philosophy, politics, science, art, etc., at variance with those generally accepted as authoritative."

The use of the term "heresy" in the context of Christianity is less common today, with some notable exceptions: see for example Rudolf Bultmann and the "character" of debates over ordination of women and gay priests. Popular imagination relegates "heresy" to the Middle Ages,Fact: date=June 2007 when the Church's power in Europe was at its height, but the case of the scholar and humanist Giordano Bruno was not the last execution for heresy. Heresy remained an officially punishable offense in Roman Catholic nations until the late 18th century. In Spain, heretics were prosecuted and punished during the Counter-Enlightenment movement of the restoration of the monarchy there after the Napoleonic Era.