It is early in the fourth century AD, and Christianity has become a religion in search of a theology. Roman persecution has ended, but doctrinal debates threaten to tear the church apart as the early church fathers strive to solve a mystery inherited from their apostolic tradition: how both Father and Son might be thought of as God, and yet as distinct, without doing violence to the tenet that God is One. Enter Arius, a Libyan priest who comes to Alexandria to preach an answer: that the Son of God is a created being of a different substance than the Father, and not fully divine. When the...