The Arian controversy was a series of Christian theological disputes that arose between Arius and Athanasius of Alexandria, two Christian theologians from Alexandria, Egypt. The most important of these controversies concerned the substantial relationship between God the Father and God the Son.
The deep divisions created by the disputes were an unfortunate and ironic consequence of Emperor Constantine’s efforts to unite Christianity and establish a single, imperially approved version of the faith during his reign. These disagreements divided the Roman Church into two opposing theological factions for over 55 years, from the time before the First Council of Nicaea in 325 until after the First Council of Constantinople in 381. There was no formal resolution or formal schism, though the Trinitarian faction ultimately gained the upper hand in the imperial Church; outside the Roman Empire this faction was not immediately so influential. Arianism continued to be preached inside and outside the Empire for some time (without the blessing of the Empire) but eventually it mostly died out. The modern Roman Catholic Church and the Eastern Orthodox Church, as well as most other modern Christian sects have generally followed the Trinitarian formulation, though each has its own specific theology on the matter.