Christianity spread, at least at first, because it was cheap. Most religions in the Roman empire, especially the fashionable mystery cults, were expensive to join, and demanded expensive sacrifices to be paid for. ( Mithras demanded bulls, for example. ) Judaism was not much better, and also demanded that you self mutilate, and obey strict diet laws.

With Christianity there was none of that, all it asked was a bit of bread and a sip of wine, which were so cheap they would give them to you, and you were in. Christianity was the religion Wallmart would sell you, in a shrink wrap.