The Council of Nicaea was a pivotal moment in the history of Christianity. Convened in 325 CE in the city of Nicaea (now Iznik, Turkey), the council was the first ecumenical council of the Christian Church. It was convened by the Roman Emperor
From Wikipedia
The First Council of Nicaea was a council of Christian bishops convened in the Bithynian city of Nicaea by the Roman Emperor Constantine I, also known as the First Ecumenical Council. It met from May until the end of July 325.