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Around the general time that Christianity was exploding into the Roman Empire, most religions weren't really into the whole missionary thing. This is a big part of why Christianity spread so well.
But it wasn't the only one. I know of two other religions existing in the Roman Empire at this time which did their fair share of proselytising: the Cult of Isis and the Cult of Mithras (which I believe was mostly peddled to soldiers).
How might one of these faiths, or another proselytising faith, have come to predominance in the way Christianity did OTL? What would have been the effects on the history of the Roman Empire, and indeed the world, if one of these faiths had been dominant instead of Christianity?