On the first Sunday after the Ascension of the Lord, the Church remembers the god-bearing fathers of the First Ecumenical Council. Here is a brief summary of the 1 st ecumenical council taken from the The Law of God by Archpriest Seraphim Slobodsky.
The First Ecumenical Council was convened in 325 A.D., in the city of Nicea, under the Emperor Constantine I. This Council was called because of the false doctrine of the Alexandrian priest Arius, who rejected the Divine nature and pre-eternal birth of the second person of the Holy Trinity, namely the Divine Son of God the Father, and taught that the Son of God is only the highest creation.
318 bishops participated in this Council, among whom were St. Nicholas the Wonderworker, St. James, bishop of Nisibis, St. Spiridon of Tremithus, and St. Athanasius, who was at that time a deacon.
The Council condemned and repudiated the heresy of Arius and affirmed the immutable truth, the dogma that the Son of God is true God, born of God the Father before all ages, and is eternal, as is God the Father; He was begotten, and not made, and is of one essence with God the Father. In order that all Orthodox Christians may know exactly the true teaching of the faith, it was clearly and concisely summarized in the first of seven sections of the Creed, or Symbol of Faith.
At this Council, it was resolved to celebrate Pascha on the first Sunday after the first full moon following the spring equinox, after the Jewish Passover. It also determined that priests should be married, and it established many other rules or canons.