Constantine and Licinius (the emperor of the East) met in Milan in Feb. 313 and agreed on religious toleration. Later, Licinius issued an edict of toleration (June 313) in Nicomedia.
Constantine was already not only tolerating Christianity in his half of the empire but actively favoring it. But he was also trying to compel the Donatists to submit to the Catholics in North Africa (that was a schism over who was the rightful bishop of Carthage rather than a question of heresy, at least at the outset).