But Christianity did not become the 'official' religion of Rome until 379AD under the rule of Theodosius I, some 50 years AFTER the Council of Nicaea. Constantine remained Pagan until his baptism on his death bed.
There are remarkable similarities between the story of Jesus and many of the Pagan religions of that time ie: virgin birth of Mithras on 25th December, Dionysus celebrated a 'Last Supper' with his 12 disciples before his death, Mithras was attended after his birth by three shepherds, Attis died and was ressurected between March 23rd and March 25th.. Easter! "He who will not eat of my body and drink of my blood, so that he will be made on with me and I with him, the same shall not know salvation." is an ancient inscription to Mithras... the list goes on!
One of the main differences between the pagan and Christain religions, however, is that the stories contained within the Pagan religions were taught as a way for the uninitiated to understand rather than as the 'gospel truth'. As one became more initiated into the religions 'gnosis', rather than the stories, was at the root of the faith.
Is it not possible, indeed quite probable then, that what happened at the Council of Nicaea was a unification of religion. An amalgamation of Christianity into Pagan thought? After all, this had happened in the past during the Roman Empire; for example, the cult of Serapis!