Arius (250 or 256–336) is a Christian presbyter in Alexandria, Egypt, of Libyan origins.

His teachings about the nature of the Godhead, which emphasize the Father's divinity over the Son,and his opposition to the Athanasian or Trinitarian Christology, make him a controversial figure in the First Council of Nicea, convened by Roman Emperor Constantine in CE 325.

After Emperor Constantine legalizes and formalizes the Christianity of the time in the Roman Empire, the newly recognized catholic Church seeks to unify and clarify its theology.

Trinitarian Christians, including Athanasius, use Arius and Arianism as epithets to describe those who disagree with their doctrine of co-equal Trinitarianism, a Christology representing God the Father and Son (Jesus of Nazareth) as "of one essence" (consubstantial) and coeternal.

Although virtually all positive writings on Arius' theology have been suppressed or destroyed, negative writings describe Arius' theology as one in which there was a time before the Son of God, when only God the Father existed.

Despite concerted opposition, 'Arian', or nontrinitarian Christian churches persisted throughout Europe and North Africa, in various Gothic and Germanic kingdoms, until suppressed by military conquest or voluntary royal conversion between the fifth and seventh centuries.

Although "Arianism" suggests that Arius was the originator of the teaching that bears his name, the debate over the Son’s precise relationship to the Father did not begin with him.

This subject had been discussed for decades before his advent; Arius merely intensified the controversy and carried it to a Church-wide audience, where other "Arians" such as Eusebius of Nicomedia would prove much more influential in the long run.

Eusebius of Nicomedia should not be confused with Eusebius of Caesarea (Bishop of Caesarea of Palestine), a renowned church father, a church historian and eulogistic biographer of Roman Emperor Constantine.

Eusebius of Caesarea is possibly one of the drafters of the Nicean creed.

In fact, some later "Arians" disavowed that moniker, claiming not to have been familiar with the man or his specific teachings.

However, because the conflict between Arius and his foes brought the issue to the theological forefront, the doctrine he proclaimed—though not originated by him—is generally labeled as "his".