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The message of Christianity is excellent for subjugating populations, giving them purpose & hope, keeping people busy & out of trouble. I think that all sounds very appealing to rulers.
And the roman empire was not able to do that already? From my understanding mythologically/spiritually the Roman empire was perfectly settled with what it had before in terms of political power. Incidentally the roman empire declined and fell apart in the centuries after accepting Christianity.
And before the Roman empire there were the Egyptians. They seemed to fare much better in terms of power with their idol worship than later when they embraced Abrahamic religions, yet they did.