Arianism, the doctrine taught by Arius (?–336?), a priest of Alexandria, Egypt. He taught that Jesus Christ was not coeternal with God but was created by God and thus was inferior to him. This doctrine struck at the doctrine of the Trinity and divided the Church. Emperor Constantine the Great called the Council of Nicaea in 325 to settle the issue. Under the leadership of Saint Athanasius the council declared that Christ is coeternal with God and "of the same substance" with him. Arius thus became a heretic and was banished for a time. The controversy was not settled until the Council of Constantinople in 381 reaffirmed the decision of Nicaea.

The heresy was stamped out in the Roman Empire, but many Germanic tribes were converted to Arian Christianity. When Germanic invaders overran the empire they brought Arianism back with them. Gradually they were converted to Catholic Christianity, and the sect died out. Arian ideas appeared again in modern times, and helped pave the way for Unitarianism.