Etymology and definition
The word "orthodoxy" comes from Greek ὀρθοδοξία orthodoxía "right opinion".
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.
The first known usage of the term 'heresy' in a civil legal context was in 380 by the "Edict of Thessalonica" of Theodosius I. Prior to the issuance of this edict, the Church had no state-sponsored support for any particular legal mechanism to counter what it perceived as 'heresy'.
Orthodoxy is adherence to correct or accepted creeds, especially in religion.
Heresy is used today with reference to in Christianity 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.
Since the time of the apostles, the term anathema has come to mean a form of extreme religious sanction beyond excommunication, known as major excommunication. The earliest recorded instance of the form is in the Council of Elvira (c. 306), and thereafter it became the common method of cutting off heretics.
In the fifth century, a formal distinction between anathema and excommunication evolved, where excommunication entailed cutting off a person or group from the rite of Eucharist and attendance at worship, while anathema meant a complete separation of the subject from the Church.