A heresiarch (also hæresiarch, according to the Oxford English Dictionary; from the Greek: αἱρεσιάρχης, hairesiárkhēs, literally "heresy chief") is a founder or leader of a heretical doctrine or movement, as considered by those who claim to maintain an orthodox religious tradition or doctrine. For example, according to traditional Roman Catholic doctrine, King Henry VIII of England was a heresiarch, as he influenced the Archbishop of Canterbury to break away from the Catholic Church to form the Church of England, and so were the other Protestant reformers such as Martin Luther and John Calvin. The miller Domenicio Scandella (called Menocchio) of Carlo Ginzburg's The Cheese and the Worms was also declared a heresiarch by the Inquisition judges.
The first official heresy of the Christian church, Arianism, was created by heresiarch Arius. It taught that Jesus was of lesser substance than God, and was rebuked by Constantine's First Ecumenical Council of Nicea, which asserted that Jesus and God the Father were "cosubstantial", i.e. of the same divine substance.