[4][9] The major impetus for the calling of the Council of Nicaea arose in a theological dispute among the Christian clergy of Alexandria concerning the nature of Jesus, his origin, and relation to God the Father.

[12] Arius' teachings are known partially from a few pieces of his writing which survive, but principally from his opponents, primarily Alexander and Athanasius of Alexandria.

[15][16] Arius accused Alexander of following the teachings of Sabellius, who taught that the Father, Son, and Holy Spirit were one person, rather than the view held throughout the east that they were distinct.

Hosius apparently then presided over a synod at Alexandria concerning the date of Easter, before calling a council of Eastern bishops in Antioch.

This council endorsed Alexander's position and issuing a statement of faith that held that the Son was "begotten not from non-existence, but from the Father, not as made, but as genuine product" and contained anathemas against Arius.

[33] The few western attendees were Hosius, Caecilianus of Carthage, Nicasius of Die, Marcus of Calabria, Domnus of Pannonia, and Victor and Vicentius, two presbyters representing Bishop Sylvester of Rome.

[36] Constantine opened the council with a formal entrance after the bishops arrived, with Eusebius describing him as "like some heavenly angel of God, his bright mantle shedding lustre like beams of light, shining with the fiery radiance of a purple robe, and decorated with the dazzling brilliance of gold and precious stones."

[42] Church councils at the time were modeled after the proceedings of the Roman Senate, with the presiding officer having a large degree of control, and participants speaking in turn based on hierarchy.

[43] Probably the first matter considered was the status of Eusebius of Caesarea and the other bishops excommunicated at Antioch, as this would determine whether they could participate in the rest of the council.

[45] A statement of faith based on earlier creeds was drafted (possibly by a smaller committee), and each line was debated by the council.

[46] In addition to the Arian question, the council also considered the calculation of Easter, and adopted the Roman and Alexandrian method over the objection of several eastern bishops.

[54] Historically significant as the first effort to attain consensus in the church through an assembly representing all of Christendom,[55] the council was the first occasion where the technical aspects of Christology were discussed.

But as for those who say, There was when He was not, and, Before being born He was not, and that He came into existence out of nothing, or who assert that the Son of God is of a different hypostasis or substance, or created, or is subject to alteration or change – these the Catholic and apostolic Church anathematizes.

Some distinctive elements in the Nicene Creed, perhaps from the hand of Hosius of Cordova, were added, some specifically to counter the Arian point of view.

Nor did the Council decree that Easter must never coincide with Nisan 15 (the first Day of Unleavened Bread, now commonly called "Passover") in the Hebrew calendar.

The Finnish Orthodox Church explains, "According to the definition of the Council of Nicaea in 325, Pascha is celebrated on the first Sunday after the full moon following the vernal equinox, but always after the Jewish Passover.

[73] In the event of the death of a non-Melitian bishop or ecclesiastic, the vacant see might be given to a Melitian, provided he was worthy and the popular election were ratified by Alexander.

[74] The Council promulgated twenty new church laws, called canons (though the exact number is subject to debate), that is, rules of discipline.

The twenty as listed in the works of Nicene and Post-Nicene Fathers are as follows:[75] In the short-term, the Council did not completely solve the problems it was convened to discuss, and a period of conflict and upheaval continued for some time.

Constantine died the next year, after finally receiving baptism from Arian Bishop Eusebius of Nicomedia, and "with his passing the first round in the battle after the Council of Nicaea was ended".

When first informed of the unrest in Alexandria due to the Arian disputes, he was "greatly troubled" and, "rebuked" both Arius and Bishop Alexander for originating the disturbance and allowing it to become public.

[83] Aware also of "the diversity of opinion" regarding the celebration of Easter and hoping to settle both issues, he sent the "honored" Bishop Hosius of Cordova (Hispania) to form a local church council and "reconcile those who were divided".

[84] Constantine assisted in assembling the council by arranging that travel expenses to and from the bishops' episcopal sees, as well as lodging at Nicaea, be covered out of public funds.

[85] In addressing the opening of the council, he "exhorted the Bishops to unanimity and concord" and called on them to follow the Holy Scriptures with: "Let, then, all contentious disputation be discarded; and let us seek in the divinely-inspired word the solution of the questions at issue.

Respecting the clerical decision, and seeing the threat of continued unrest, Constantine also pronounced civil judgement, banishing them into exile.

In Nicaea, questions regarding the Holy Spirit were left largely unaddressed until after the relationship between the Father and the Son was settled around the year 362.

Despite Constantine's sympathetic interest in the Church, he was not baptized until some 11 or 12 years after the council, putting off baptism as long as he did so as to be absolved from as much sin as possible.

It involves five different arguments "drawn respectively from the grammatical structure of the sentence, from the logical sequence of ideas, from Catholic analogy, from comparison with the process of formation of the Byzantine Patriarchate, and from the authority of the ancients"[100] in favor of an alternative understanding of the canon.

[102][103] This geographic issue related to Canon 6 was highlighted by Protestant writer Timothy F. Kauffman, as a correction to the anachronism created by the assumption that each bishop was already presiding over a whole diocese at the time of the council.

Rome's jurisdiction within Italy had been defined in terms of several of the city's adjacent provinces since Diocletian's reordering of the empire in 293, as the earliest Latin version of the canon indicates.