The First Council of Nicaea was held in Nicaea in Bithynia ( in present-day Turkey ) in 325 . The Roman Emperor Constantine I called the bishops of the Roman Empire to the first ecumenical conference of the early Christian Church . The First Council of Nicaea was a council of Christian bishops convened in Nicaea in Bithynia ( present-day Ä znik in Turkey ) by the Roman Emperor Constantine I in A.D. 325 .