Arianism, in Christianity, is a Christological concept that asserts that Jesus Christ is the Son of God who was begotten by God the Father at a point in time, is distinct from the Father and is therefore subordinate to the Father. Arian teachings were first attributed to Arius (c. AD 256–336), a Christian presbyter in Alexandria, Egypt. The teachings of Arius and his supporters were opposed to the theological views held by Homoousian Christians, regarding the nature of the Trinity and the nature of Christ. The Arian concept of Christ is that the Son of God did not always exist but was begotten by God the Father.
There was a dispute between two interpretations (Arianism and Homoousianism) based upon the theological orthodoxy of the time, both of them attempted to solve its theological dilemmas. So there were, initially, two equally orthodox interpretations which initiated a conflict in order to attract adepts and define the new orthodoxy. Homoousianism was formally affirmed by the first two Ecumenical Councils. The Ecumenical First Council of Nicaea of 325 deemed it to be a heresy. All mainstream branches of Christianity now consider Arianism to be heterodox and heretical. All mainstream branches of Christianity now consider Arianism to be heterodox and heretical.
According to Everett Ferguson, "The great majority of Christians had no clear views on the Trinity and did not understand what was at stake in the issues." At the regional First Synod of Tyre in 335, Arius was exonerated. Constantine the Great was baptized by the Arian bishop Eusebius of Nicomedia. After the deaths of both Arius and Constantine, Arius was again anathemised and pronounced a heretic again at the Ecumenical First Council of Constantinople of 381. The Roman Emperors Constantius II (337–361) and Valens (364–378) were Arians or Semi-Arians, as was the first King of Italy, Odoacer (433?–493), and the Lombards till the 7th century.
Arianism is also used to refer to other nontrinitarian theological systems of the 4th century, which regarded Jesus Christ—the Son of God, the Logos—as either a begotten being (as in Arianism proper and Anomoeanism) or as neither uncreated nor created in the sense other beings are created (as in Semi-Arianism)....LESS