a Christian heresy first proposed early in the 4th century by the Alexandrian presbyter Arius. It affirmed that Christ is not truly divine but a created being. Arius' basic premise was the uniqueness of God, who is alone self-existent and immutable; the Son, who is not self-existent, cannot be God. Because the Godhead is unique, it cannot be shared or communicated, so the Son cannot be God. Because the Godhead is immutable, the Son, who is mutable, being represented in the Gospels as subject to growth and change, cannot be God. The Son must, therefore, be deemed a creature who has been called into existence out of nothing and has had a beginning. Moreover, the Son can have no direct knowledge of the Father since the Son is finite and of a different order of existence.

According to its opponents, especially the bishop Athanasius (Athanasius, Saint), Arius' teaching reduced the Son to a demigod, reintroduced polytheism (since worship of the Son was not abandoned), and undermined the Christian concept of redemption since only he who was truly God could be deemed to have reconciled man to the Godhead.

The controversy seemed to have been brought to an end by the Council of Nicaea (Nicaea, Council of) (AD 325), which condemned Arius and his teaching and issued a creed to safeguard orthodox Christian belief. This creed states that the Son is homoousion tō Patri (homoousios) (“of one substance with the Father”), thus declaring him to be all that the Father is: he is completely divine. In fact, however, this was only the beginning of a long-protracted dispute.

From 325 to 337, when the emperor Constantine died, the Arian leaders, exiled after the Council of Nicaea, tried by intrigue to return to their churches and sees and to banish their enemies. They were partly successful.

From 337 to 350 Constans (Constans I), sympathetic to the orthodox Christians, was emperor in the West, and Constantius II, sympathetic to the Arians, was emperor in the East. At a church council held at Antioch (Antioch, Council of) (341), an affirmation of faith that omitted the homoousion clause was issued. Another church council was held at Sardica (modern Sofia) in 342, but little was achieved by either council.

In 350 Constantius became sole ruler of the empire, and under his leadership the Nicene party (orthodox Christians) was largely crushed. The extreme Arians then declared that the Son was “unlike” (anomoios) the Father. These anomoeans (Anomoean) succeeded in having their views endorsed at Sirmium in 357, but their extremism stimulated the moderates, who asserted that the Son was “of similar substance” (homoiousios) with the Father. Constantius at first supported these homoiousians but soon transferred his support to the homoeans (Homoean), led by Acacius, who affirmed that the Son was “like” (homoios) the Father. Their views were approved in 360 at Constantinople (Constantinople, Council of), where all previous creeds were rejected, the term ousia (“substance,” or “stuff”) was repudiated, and a statement of faith was issued stating that the Son was “like the Father who begot him.”

After Constantius' death (361), the orthodox Christian majority in the West consolidated its position. The persecution of orthodox Christians conducted by the (Arian) emperor Valens (364–378) in the East and the success of the teaching of Basil the Great of Caesarea, Gregory of Nyssa, and Gregory of Nazianzus led the homoiousian majority in the East to realize its fundamental agreement with the Nicene party. When the emperors Gratian (367–383) and Theodosius I (379–395) took up the defense of orthodoxy, Arianism collapsed. In 381 the second ecumenical council met at Constantinople. Arianism was proscribed, and a statement of faith, the Nicene Creed, was approved.

Although this ended the heresy in the empire, Arianism continued among some of the Germanic tribes to the end of the 7th century. In modern times some Unitarians (Unitarianism and Universalism) are virtually Arians in that they are unwilling either to reduce Christ to a mere human being or to attribute to him a divine nature identical with that of the Father. The Christology of Jehovah's Witnesses (Jehovah's Witness), also, is a form of Arianism; they regard Arius as a forerunner of Charles Taze Russell, the founder of their movement.