The First Ecumenical Council is convened in Nicea by Roman Emperor Constantine I. The Council, made up of a large number of delegates from most all Christian churches in the world at that time, addressed important issues that would form the backbone of modern Christianity.
One of the main disputes at the Council came from Arius. His ideas on the nature of God the Father and the Son differed considerably from other more western views. Arius preached that God the Father was just as divine as God the Son, but that God the Father came before the Son. Arius believed that God the Father was more infinitely powerful than God the Son.
The Council at Nicea ruled against these ideas and declared the Nicene Creed, deciding that all parts of the Holy Trinity are one and the same and equally powerful and infinite. The Nicene Creed represents the first unified Christian doctrine. The Apostle's Creed used extensively in many churches today is closely related to the concepts of the Nicene Creed.
The First Council of Nicea also set the method of calculating the official date for the celebration of Easter.