Arian controversy
The Arian controversy was a Christological dispute that began in Alexandria between the followers of Arius, the Arians; the followers of St. Alexander of Alexandria, known as homoousians; and a third group, known as homoiousians. Alexander and his followers believed that the Son was of the same substance as the Father, co-eternal with him. The Arians believed that they were different and that the Son, though he may be the most perfect of creations, was only a creation. Homoiousians tried to make a compromise position, saying that the Father and the Son were of similar substance.
Much of the debate hinged on the difference between being "born" or "created" and being "begotten". Arians saw these as the same; followers of Alexander did not. Indeed, the exact meaning of many of the words used in the debates at Nicaea were still unclear to speakers of other languages; Greek words like "essence" (ousia), "substance" (hypostasis), "nature" (physis), "person" (prosopon) bore a variety of meanings drawn from pre-Christian philosophers, which could not but entail misunderstandings until they were cleared up. The word homoousia, in particular, was initially disliked by many bishops because of its associations with Gnostic heretics (who used it in their theology), and because it had been condemned at the 264-268 Synods of Antioch.
Homoousians believed that to follow the Arian view destroyed the unity of the Godhead, and made the Son unequal to the Father, in contravention of the Scriptures ("The Father and I are one", John 10:30). Arians, on the other hand, believed that since God the Father created the Son, he must have emanated from the Father, and thus be lesser than the Father, in that the Father is eternal, but the Son was created afterward and, thus, is not eternal. The Arians likewise appealed to Scripture, quoting verses such as John 14:28: "the Father is greater than I". Homoousians countered the Arians' argument, saying that the Father's fatherhood, like all of his attributes, is eternal. Thus, the Father was always a father, and that the Son, therefore, always existed with him.
The Council declared that the Father and the Son are of the same substance and are co-eternal, basing the declaration in the claim that this was a formulation of traditional Christian belief handed down from the Apostles. This belief was expressed in the Nicene Creed.