The First Œcumenical Council of Holy Mother Church was convened in the year of our Lord 325, as the Christian world was being attacked by Arianism, a heresy which denied the Divinity of our Lord Jesus Christ.

318 Bishops from what was then all of Christendom gathered at Nicæa, modern Iznik, across the narrow strait of the Bosphorus from the great city of Constantinople (now Istanbul), seventy-five miles inland in what is now Asiatic Turkey. They assembled from May or June until the 25th of August, the Council being opened by Emperor Constantine (Pope St. Sylvester I was represented by two legates, but did not actually attend the Council).

The Council defined some very important points of Christian Doctrine, beginning with a symbol, or creed, containing a summary of the Trinitarian Faith handed on from the Apostles. This credo would later be expanded by the First Council of Constantinople into the form that is said at Holy Mass today. They then proceeded to define 20 Canons, most dealing with the Clergy. The only other document related to the Council is the Letter to the Egyptian Bishops who could not make it to the Council. It summarizes the main decisions of the Council.