The Council of Nicaea, with Arius depicted as defeated by the council, lying under the feet of Emperor Constantine. The Hagia Sophia is in the background of the icon. (en)
Arianism, the nature of Christ, celebration of Passover, ordination of eunuchs, prohibition of kneeling on Sundays and from Easter to Pentecost, validity of baptism by heretics, lapsed Christians, sundry other matters (en)