The first Ecumenical Council was presided over Emperor Constantine I and held in Izmit, Turkey . The council was convened to address the Arian heresy, which denied the divinity of Christ.
It is believed to have begun on May 20, 325 AD and lasted through July.
The Council formulated the Nicene Creed, a declaration of faith affirming that Jesus is "true God from true God," "consubstantial with the Father," and other core tenets of Christian belief.