Kind of the reason Christianity got a foothold is that belief in the traditional Greco-Roman gods was dropping in late antiquity. Lots of new religions were beg

Kind of the reason Christianity got a foothold is that belief in the traditional Greco-Roman gods was dropping in late antiquity. Lots of new religions were beginning to pop up -- Christianity, the cult of Isis (which was based on a Egyptian goddess but the way they worshiped her was new), the cult of Mithras. At one way, the death of traditional paganism can be seen as a win for rationality, although subsequent events suggests that getting rid of religion entirely is difficult because new religions tend to fill the vacuum left by the old making no net increase in overall rationality.