Arius (AD 250 or 256 – 336) was a Christian presbyter (an elder of the congregation in the early Christian church) from Alexandria, Egypt. His teachings about the nature of the Godhead, which emphasized the Father's Divinity over the Son, and his opposition to the Trinitarian Christology, made him a controversial figure in the Church. After Emperor Constantine legalized and formalized Christianity as the religion for the entire Roman Empire, Church officials sought to unify and clarify its theology. To do so, the Emperor convened the First Council of Nicea in 325 A.D.
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, where only God the Father existed. Despite concerted opposition, 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 he began it; 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. 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".
Arius taught that God the Father and the Son did not exist together eternally. He taught that Jesus did not exist before he was created by God. Thus, because Jesus was created by God, he was inferior to God. Arius and his followers appealed to Bible verses such as John 14:28 where Jesus says that the father is "greater than I."
Auxentius, a 4th-century Arian bishop of Milan, gives the clearest picture of Arian beliefs on the nature of the Trinity: God the Father ("unbegotten"), always existing, was separate from the lesser Jesus Christ ("only-begotten"), born before time began and creator of the world. The Father, working through the Son, created the Holy Spirit, who was subservient to the Son as the Son was to the Father. The Father was seen as "the only true God."
[However, I myself can imagine a theory that could have been put forth in this debate: God (male) and the Holy Spirit (female) married. They decided to have a son on earth who would be both human and divine; so God impregnated the Holy Spirit; then the Holy Spirit went into the womb of the virgin Mary, through whom Jesus was delivered as both human and Divine.]
Of all the various disagreements within the Christian Church, the Arian controversy has held the greatest force and power of theological and political conflict, with the possible exception of the Protestant Reformation. The conflict between Arian’s beliefs (nontrinitarianism) and Tertullian’s beliefs (trinitarianism) was the first major doctrinal confrontation in the Church after the legalization of Christianity by the Roman Emperor Constantine I.
Such a deep controversy within the Church could not have materialized in the 3rd and 4th centuries without some significant historical influences providing the basis for the Arian doctrines. Of the roughly three hundred bishops in attendance at the Council of Nicea, the majority of them signed the Nicene Creed, which officially established the doctrine of the trinity for the Church. However, to minimize the extent of Arianism ignores the fact that extremely prominent Emperors such as Constantius II, and Valens were Arians, as well as prominent Gothic, Vandal and Lombard warlords both before and after the fall of the Western Roman Empire, and that none of these groups was out of the mainstream of the Roman Empire in the 4th century. However, after the Nicene Creed officially settled the debate with a great majority holding to the Trinitarian position, the Arian position was officially declared to be heterodox—that is, not in agreement with accepted beliefs.
While Arianism continued to for several decades even within the family of the Emperor, the Imperial nobility, and higher-ranking clergy, in the end it was Trinitarianism which prevailed in the Roman Empire at the end of the 4th century.