What had begun in c.196 C.E. as a challenge to the Quartodeciman practice of Passover/Unleavened Bread (System A) by Victor, bishop of Rome, was finally granted full authority throughout the Roman empire at the behest of Emperor Constantine.
Constantine convened the Council of Nicaea in 325 C.E.
Authority from Constantine
One of the results of this conference was the declaration by Constantine that the Hasidic view for the seven days of Unleavened Bread, as instituted by Pope Victor, was the correct system under the Torah.