The emperor Galerius puts a stop to the Christian persecutions in 311 by issuing the Edict of Toleration. Two years later, his successor Constantin declares freedom of religion with his Edict of Milan. Read both edicts here. Constantin becomes the first Christian emperor of Rome when he is baptised on his deathbed in 337.
“… no one whatsoever should be denied the opportunity to give his heart to the observance of the Christian religion, of that religion which he should think best for himself…”
– Constantin, The Edict of Milan