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First Council of Nicaea

The Christological debate could no longer be contained within the Alexandrian diocese. By the time Bishop Alexander finally acted against Arius, Arius's doctrine had spread far beyond his own see; it had become a topic of discussion—and disturbance—for the entire Church. The Church was now a powerful force in the Roman world, with Emperors Licinius and Constantine I having legalized it in 313 through the Edict of Milan. Emperor Constantine had taken a personal interest in several ecumenical issues, including the Donatist controversy in 316, and he wanted to bring an end to the Christological dispute. To this end, the emperor sent Hosius, bishop of Córdoba to investigate and, if possible, resolve the controversy. Hosius was armed with an open letter from the Emperor: "Wherefore let each one of you, showing consideration for the other, listen to the impartial exhortation of your fellow-servant." However, as the debate continued to rage despite Hosius's efforts, Constantine in AD 325 took an unprecedented step: he called a council to be composed of church prelates from all parts of the empire to resolve this issue, possibly at Hosius's recommendation.

"Around 250–300 attended, drawn almost entirely from the eastern half of the empire." Pope Sylvester I, himself too aged to attend, sent two priests as his delegates. Arius himself attended the council, as did his bishop, Alexander. Also there were Eusebius of Caesarea, Eusebius of Nicomedia and the young deacon Athanasius, who would become the champion of the Trinitarian view ultimately adopted by the council and spend most of his life battling Arianism. Before the main conclave convened, Hosius initially met with Alexander and his supporters at Nicomedia. The council was presided over by the emperor himself, who participated in and even led some of its discussions.

At this First Council of Nicaea, 22 bishops, led by Eusebius of Nicomedia, came as supporters of Arius. Nonetheless, when some of Arius's writings were read aloud, they are reported to have been denounced as blasphemous by most participants. Those who upheld the notion that Christ was co-eternal and consubstantial with the Father were led by the bishop Alexander. Athanasius was not allowed to sit in on the Council because he was only an arch-deacon. However, Athanasius is seen as doing the legwork and concluded (according to Bishop Alexander's defense of Athanasian Trinitarianism and also according to the Nicene Creed adopted at this Council) that the Son was of the same essence (homoousios) with the Father (or one in essence with the Father), and was eternally generated from that essence of the Father. Those who instead insisted that the Son of God came after God the Father in time and substance were led by Arius the presbyter. For about two months, the two sides argued and debated, with each appealing to Scripture to justify their respective positions. Arius argued for the supremacy of God the Father, and maintained that the Son of God was simply the oldest and most beloved creature of God, made from nothing, because of being the direct offspring. Arius taught that the pre-existent Son was God's first production (the very first thing that God actually ever did in his entire eternal existence up to that point), before all ages. Thus he insisted that only God the Father had no beginning, and that the Father alone was infinite and eternal. Arius maintained that the Son had a beginning. Thus, said Arius, only the Son was directly created and begotten of God; furthermore, there was a time that he had no existence. He was capable of his own free will, said Arius, and thus "were He in the truest sense a son, He must have come after the Father, therefore the time obviously was when He was not, and hence He was a finite being." Arius appealed to Scripture, quoting verses such as John 14:28: "the Father is greater than I", as well as Colossians 1:15: "the firstborn of all creation." Thus, Arius insisted that the Father's Divinity was greater than the Son's, and that the Son was under God the Father, and not co-equal or co-eternal with him.

According to some accounts in the hagiography of Nicholas of Myra, debate at the council became so heated that at one point, Nicholas struck Arius across the face. The majority of the bishops ultimately agreed upon a creed, known thereafter as the Nicene creed. It included the word homoousios, meaning "consubstantial", or "one in essence", which was incompatible with Arius's beliefs. On June 19, 325, council and emperor issued a circular to the churches in and around Alexandria: Arius and two of his unyielding partisans (Theonas and Secundus) were deposed and exiled to Illyricum, while three other supporters— Theognis of Nicaea, Eusebius of Nicomedia and Maris of Chalcedon—affixed their signatures solely out of deference to the emperor. The following is part of the ruling made by the emperor denouncing Arius's teachings with fervor.

In addition, if any writing composed by Arius should be found, it should be handed over to the flames, so that not only will the wickedness of his teaching be obliterated, but nothing will be left even to remind anyone of him. And I hereby make a public order, that if someone should be discovered to have hidden a writing composed by Arius, and not to have immediately brought it forward and destroyed it by fire, his penalty shall be death. As soon as he is discovered in this offense, he shall be submitted for capital punishment