Arianism (Koinē Greek: Ἀρειανισμός, Areianismós) is a Christological doctrine first attributed to Arius (c. AD 256–336), a Christian presbyter from Alexandria, Egypt. Arian theology holds that Jesus Christ is the Son of God,[lower-alpha 1][lower-alpha 2] who was begotten by God the Father with the difference that the Son of God did not always exist but was begotten/made[lower-alpha 3] before "time" by God the Father;[lower-alpha 4] therefore, Jesus was not coeternal with God the Father, but nonetheless Jesus began to exist outside time as time applies only to the creations of God.[lower-alpha 5]
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Arius' trinitarian theology, later given an extreme form by Aetius and his disciple Eunomius and called anomoean ("dissimilar"), asserts a total dissimilarity between the Son and the Father. Arianism holds that the Son is distinct from the Father and therefore subordinate to him. The term Arian is derived from the name Arius; it was not what the followers of Arius' teachings called themselves, but rather a term used by outsiders. The nature of Arius's teachings and his supporters were opposed to the theological doctrines held by Homoousian Christians, regarding the nature of the Trinity and the nature of Christ.
There was a controversy between two interpretations of Jesus's divinity (Homoousianism and Arianism) based upon the theological orthodoxy of the time, one trinitarian and the other also a derivative of trinitarian orthodoxy, and both of them attempted to solve its respective theological dilemmas. Homoousianism was formally affirmed by the first two ecumenical councils; since then, Arianism has always been condemned as "the heresy or sect of Arius". As such, all mainstream branches of Christianity now consider Arianism to be heterodox and heretical. Trinitarian (Homoousian) doctrines were vigorously upheld by Patriarch Athanasius of Alexandria, who insisted that Jesus (God the Son) was "same in being" or "same in essence" with God the Father. Arius stated: "If the Father begat the Son, then he who was begotten had a beginning in existence, and from this it follows there was a time when the Son was not." The ecumenical First Council of Nicaea of 325, convened by Emperor Constantine to ensure church unity, declared Arianism to be a heresy. According to Everett Ferguson, "The great majority of Christians had no clear views about the nature of the Trinity and they did not understand what was at stake in the issues that surrounded it."
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 creature of a similar or different substance to that of the Father, but not identical (as Homoiousian and Anomoeanism) or as neither uncreated nor created in the sense other beings are created (as in semi-Arianism).