Robby, you are wrong, Constantine just in 313 with the Edict of Milan made the Empire religion neutral
his predecessor Galerius ended the persecutions of Christians in 311
And it was only Theodosius I, Flavius Theodosius Augustus who, on 27 February 380 declared the Catholic Church the only legitimate Imperial religion
It is our desire that all the various nation which are subject to our clemency and moderation, should continue to the profession of that religion which was delivered to the Romans by the divine Apostle Peter, as it has been preserved by faithful tradition and which is now professed by the Pontiff Damasus and by Peter, Bishop of Alexandria, a man of apostolic holiness. According to the apostolic teaching and the doctrine of the Gospel, let us believe in the one diety of the father, Son and Holy Spirit, in equal majesty and in a holy Trinity. We authorize the followers of this law to assume the title Catholic Christians;
So, this wasn't Constantine
De facto usually beats de jure,