http://www.newadvent.org/cathen/11044a.htm
First Ecumenical Council of the Catholic Church, held in 325 on the occasion of the heresy of Arius (Arianism). As early as 320 or 321 St. Alexander, Bishop of Alexandria, convoked a council at Alexandria at which more than one hundred bishops from Egypt and Libya anathematized Arius. The latter continued to officiate in his church and to recruit followers. Being finally driven out, he went to Palestine and from there to Nicomedia. . . .