Constantine the Great, who along with Licinius had decreed toleration of Christianity in the Roman Empire by what is commonly called the "Edict of Milan", and was the first Roman Emperor baptized, set precedents for later policy. By Roman law the Emperor was Pontifex Maximus, the high priest of the College of Pontiffs (Collegium Pontificum) of all recognized religions in ancient Rome. To put an end to the doctrinal debate initiated by Arius, Constantine called the first of what would afterwards be called the ecumenical councils and then enforced orthodoxy by Imperial authority.
Who was the first Roman Emporor that was baptized?
Constantine the Great