First Council of Nicaea
The purpose of the council was to resolve disagreements in the Church of Alexandria over the nature of Jesus in relationship to the Father; in particular, whether Jesus was of the same substance as God the Father or merely of similar substance. St. Alexander of Alexandria and Athanasius took the first position; the popular presbyter Arius, from whom the term Arian controversy comes, took the second.  The council decided against the Arians (of the estimated 250-318 attendees, all but 2 voted against Arius). But many of the Eastern bishops who were pro-Arian, were prevented from reaching the Council until after the vote had been taken.