There never was a flood of ‘Oriental religions’, as suggested, once again, by Cumont. As we have seen, there were only a few Mystery cults of Isis, and although the number of followers of Mithras in the West was considerable, it should not be overstated. As far as numbers are concerned, Mystery cults never posed a serious threat to emerging Christianity. There are only a few possible references to pagan Mystery cults in the New Testament, which should not surprise us, as interest in the Mysteries flourished most in the second century AD. It is in that period that we start to notice a shared interest by both pagans and Christians in the Mysteries. Pagans seem to have been struck by similarities, but Christians stressed the differences.
The fact that initiation into the Mysteries could be a costly affair and that the Mithras cult was limited to males meant that pagan Mysteries were no competition for Christianity on the religious market, as the latter always received young and old, rich and poor, male and female into its fold. Moreover, unlike the Mysteries, Christianity was not esoteric but at first openly proclaimed its message, which was clear to all.—Initiation into the Mysteries of the Ancient World, pages 188-189