The First Council of Nicaea was a council of Christian bishops convened in the Bithynian city of Nicaea (now İznik, Bursa province, Turkey).
This ecumenical council was the first effort to attain consensus in the Church through an assembly representing all of Christendom. Hosius of Corduba, who was probably one of the Papal legates, may have presided over its deliberations 20 May.
Its main accomplishments were a settlement of the Christological issue of the divine nature of God the Son and his relationship to God the Father, the construction of the first part of the Nicene Creed, establishing uniform observance of the date of Easter, and promulgation of early canon law.
Overview – 20 May
Eastern Orthodox icon depicting the First Council of Nicaea
The First Council of Nicaea was the first ecumenical council of the Church. Most significantly, it resulted in the first uniform Christian doctrine, called the Nicene Creed. With the creation of the creed, a precedent was established for subsequent local and regional councils of Bishops (Synods) to create statements of belief.
One purpose of the council was to resolve disagreements arising from within the Church of Alexandria over the nature of the Son in his relationship to the Father: in particular, whether the Son had been ‘begotten’ by the Father from his own being, and therefore having no beginning, or else created out of nothing, and therefore have a beginning.
Another result of the council was an agreement on when to celebrate Easter, the most important feast of the ecclesiastical calendar, decreed in an epistle to the Church of Alexandria in which is simply stated:
We also send you the good news of the settlement concerning the holy pasch, namely that in answer to your prayers this question also has been resolved. All the brethren in the East who have hitherto followed the Jewish practice will henceforth observe the custom of the Romans and of yourselves and of all of us who from ancient times have kept Easter together with you.
Historically significant as the first effort to attain consensus in the church through an assembly representing all of Christendom, the Council was the first occasion where the technical aspects of Christology were discussed. Through it a precedent was set for subsequent general councils to adopt creeds and canons. This council is generally considered the beginning of the period of the First seven Ecumenical Councils in the History of Christianity.