When heresy is used today with reference to Christianity, it denotes the formal denial or doubt of a core doctrine of the Christian faith as defined by one or more of the Christian churches. It should be distinguished from both apostasy and schism, apostasy being nearly always total abandonment of the Christian faith after it has been freely accepted, and schism being a formal and deliberate breach of Christian unity and an offence against charity without being based essentially on doctrine.
The Catholic Church distinguishes between "formal heresy" and "material heresy". The former involves willful and persistent adherence to an error in matters of faith and is a grave sin for which the church applies the penalty of excommunication. "Material heresy" is the holding of erroneous opinions through no fault of one's own and is not sinful. Protestants can fall into either the first or the second group, depending on whether they are in invincible ignorance while the Eastern Orthodox are considered to be schismatic but are recognised as local churches, but "defective" (according to Dominus Iesus) and severed from the Catholic Church, which in Catholic teaching is the "only true Church".
Historical examination of heresies focuses on a mixture of theological, spiritual, and socio-political underpinnings to explain and describe their development. For example, accusations of heresy have been levelled against a group of believers when their beliefs challenged, or were seen to challenge, Church authority. Some heresies have also been doctrinally based, in which a teaching was deemed to be inconsistent with the fundamental tenets of orthodox dogma.
The study of heresy requires an understanding of the development of orthodoxy and the role of creeds in the definition of orthodox beliefs. Orthodoxy has been in the process of self-definition for centuries, defining itself in terms of its faith and changing or clarifying beliefs in opposition to people or doctrines that are perceived as incorrect. The reaction of the orthodox to heresy has also varied over the course of time; many factors, particularly the institutional, judicial, and doctrinal development of the Church, have shaped this reaction. Heresy remained an officially punishable offence 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.
The word heresy comes from haeresis, a Latin transliteration of the Greek word originally meaning choosing, choice, course of action, or in an extended sense school of thought then eventually came to denote warring factions and the party spirit by the first century. The word appears in the New Testament and was appropriated by the Church to mean a sect or division that threatened the unity of Christians. Heresy eventually became regarded as a departure from orthodoxy, a sense in which heterodoxy was already in Christian use soon after the year 100.