TD Barnes has vigorously contested popular ideas of the Edict of Milan: It was not issued in Milan and didn’t affect Italy; it didn’t legalize Christianity, which was already legal; it was not an edict.
This can leave the impression that the declaration of Licinius on June 13, 313 was no big deal. Though recognizing that Barnes is right about details, Potter thinks that it truly was “the official beginning of a new era in the relationship between church and state” (Constantine the Emperor, 148-9).
He continues, “it expresses a message of inclusion that goes far beyond the edicts of Galerius and Maximinus [both pagans who legalized Christianity in West and East, respectively]. While these two emperors had promised that they would no longer persecute the Christians for the good of the state as a whole, in this edict Licinius and Constantine state that the Christians are protected by the Highest God, who has aided them in their victories. All who worshipped the gods were therefore to be treated equally. It is a stunning assertion by a Roman emperor that freedom of thought is a good thing, and it remains such even though neither Licinius nor Constantine would always follow through on this point.”