Chrstianity was defined by Christ ... not Constantine at Nicea!
That's true: Christianity ws defined by Christ. The Council of Nicaea was convened by Constantine but he did not have the power to control its decisions, which were guided by the Holy Spirit.
Constantine was just an unbaptized catechumen, and did not have or claim the authority to arrange its conclusions. If he had, the Concil would have gone for Arianism, because Constantine inclined that way. But as it was, the Council found in favor of a Biblical, Orthodox Catholic Christology: Christ both true God and true Man, co-equal and co-eternal with the Father and the Holy Spirit.