Early Career and Defense of Nicene Orthodoxy
Around 360–361 Flavian was ordained to the priesthood by Meletius of Antioch. Working alongside Diodore of Tarsus, he defended Nicene Christianity against Arianism during the episcopate of the Arian bishop Leontius of Antioch.
According to Theodoret, it was in the meetings led by Flavian and Diodore — assembled outside the city walls — that the practice of antiphonal singing was first introduced into the services of the Church.