Where would Carangian be from?
I'm lackadaisically reading Ramsay MacMullen's Christianizing the Roman Empire, and one of the interesting points he makes is how dormant Christians seemed to be, as a public proselytizing community, during the 2nd and 3rd centuries. But it's also true that healing and sickness were key points in ancient religions. So while Christianity appears to have gained its greatest numbers of converts (normal people, not self-consciously intellectual aristocrats) through miracles of healing and exorcism, I still don't think that plague was the determinative factor.
Mithraism was really weird. If you ever see in a bookstore the 2 volume Sourcebook of Roman Religion, recently published by someone and written by someone (they're in another room, sorry), flip through the source documents/sketches for Mithraism. BTW, I think those books are pure torture to read, but they're pretty good references. Anyway, Mithraism was, by name, an offshoot of the Persian cult of Mithras, of course heavily influenced by Zoroastrianism. But the Roman cult appears to have retained no more Zoroastrian influences than, say, Christianity. And the Persian connection was hyped up by Roman adherents as a kind of exotic oriental selling point, even though Roman Mithras was rather unlike Persian Mithras.
At any rate, Mithras was a big rival, but no Mithraites (?) ever claimed that non-Mithraites would be smote by Mithras for not worshipping Him, which is what pagans found so weird about Christians.
driftwood