Constantine and Licinius got into a war against each other. We had the Battle of Adrianople. The Battle of Hellespont. Finally the Battle of Chrysopolis, and Constantine was sole Emperor. So, since Christ had delivered victory, Constantine said, “Okay, I’m a Christian now! By the way, what do you guys believe, anyway?”
To which the answer was, “Depends on who you ask. In which town. On what day.”
Constantine was a Roman emperor, and a military man. So he said, “Right. Figure it out and tell me. I’ll believe anything you say, but get it all in one sock.” He called NiceaCon One, and invited all the BNFs and SMOFs of the Christian world to have a business meeting and hammer it out.
So, all the bishops of the world went to Nicea. Depending on who you ask, there were either “more than two hundred” (Eusebius of Caesarea), or 318 (Athanasius of Alexandria). Athanasius may have been counting non-voting members since he himself was there as the secretary of Bishop Alexander of Alexandria.
Reportedly only five bishops from the Latin west attended, not including the Bishop of Rome (Pope Sylvester I — but he did send two priests as legates).
The big question was the Creed, or What Do We Believe, Anyway? (Other questions included “What Do We Do With the Christians Who Supported Licinius?”, “What Do We Do With Christians Who, Faced With Persecution, Said, ‘Sacrifice To Zeus? Hoo Boy Yeah! Me an’ Zeus, We’re Tight!’”, and “Can Guys Who Have Been Castrated Be Deacons?” But I’m going to skip all of those to get to the main event.)
In coming up with a creed, the biggest question was “What is the Nature of Christ?” One side, led by the pious and scholarly Arius, held that Jesus was the first and greatest of God’s creations (that is, essentially, Top Angel). The other side, championed by Athanasius, held that Jesus was actually Totally God Since Forever. Both sides had copies of old fanzines to support their position.
(When Constantine heard this he said, “Can’t you guys just get along? Why not agree to disagree like every other friggin’ philosopher since Plato was a pup, and get on with your lives?” to which both sides answered “No!!!!eleventy!!!” and thus Nicea.)