Constantine essentially adopted Christianity as the official roman religion and turned it to persecuting the others, it's when it went from one of the many religions of Rome to a world spanning religion.
That wasn't Constantine, that was Theodosius. Constantine just made Christianity
one of the official Roman religions. He himself was not baptized until his deathbed.
The trick Constantine played is that as emperor, he was recognized by the empire as also
pontifex maximus -- the highest priest -- of
all Roman official religions. When he recognized Christianity as an official religion of the empire, adherents of all other official religions were forced to recognize him as the
pontifex maximus of Christianity as well. That is most likely why he delayed baptism until the end of life, because that step would have required him to repudiate leadership of all the other religions.
Did Christians acknowledge him in that role? Probably not completely, but they did acquiesce to his demands on them in some significant theological issues.
Had he or someone like him never come to power you probably wouldn't be Christian.
As I mentioned, Christianity was growing despite resistance by Roman emperors, so I would argue that "someone like him" was inevitable.
That aside, the point was that he came to power via a violent seizure of power, sweeping aside the alternative emperors with military force.
To say that both he and the alternative persecution both have divine right to rule but not to be overthrown is a bit odd.
I would not argue "divine right" of any individual. As I said before, they are all tools that God picks up for His purpose and then tosses them away when He's finished with them.
