Its story
Around 318–320 Arius, presbyter of the Baucalis church in Alexandria, came into conflict with his bishop, Alexander. Arius wanted to affirm strongly that God is one, unoriginate and indivisible; therefore the Son, though prior to time and the agent of creation, must have been begotten or created by the Father's will out of nothing. His supporters summed this up as 'there was when he was not'. Arius appealed to texts such as Proverbs 8:22 ('the Lord created me at the beginning of his ways', in the Greek version) and John 14:28 ('the Father is greater than I').
The dispute spread across the East and Emperor Constantine summoned the first ecumenical council at Nicaea (325). The Nicene Creed declared the Son 'begotten, not made, consubstantial (homoousios) with the Father' and anathematized Arian formulas. Yet homoousios was not a scriptural word and many thought it smacked of modalism, so for half a century alternative formulas followed: 'like in substance' (homoiousios), 'like' (homoios) and the radical 'unlike' (anomoios) of Aetius and Eunomius. Under emperors who favored the Homoians, Athanasius of Alexandria was exiled five times.
Athanasius, Hilary of Poitiers and then the Cappadocian Fathers (Basil of Caesarea, Gregory of Nazianzus, Gregory of Nyssa) argued that if the Son is not fully God he cannot reveal the Father or save humanity: only God can unite humanity to God. The distinction between one ousia and three hypostases made it possible to accept homoousios without falling into modalism. With Emperor Theodosius, the Edict of Thessalonica (380) and the Council of Constantinople (381), Nicene faith prevailed in the Empire.
Arianism survived beyond the Empire: the missionary Ulfilas had evangelized the Goths in a Homoian form, and Visigoths, Ostrogoths, Vandals and Lombards were Arian for generations. The Visigoths of Spain embraced Nicene faith at the Third Council of Toledo (589) and the Lombards in the 7th century. Modern historians note that 'Arianism' was largely a polemical label coined by Athanasius to group diverse positions, many not directly dependent on Arius. In modern times some non-Trinitarian groups have been compared to Arianism.