This is a pretty tendentious reading of Hellenistic philosophy. “Decadent?” Come on.
The philosophical schools required quite a lot of fantastical credulity. Epicureans had to believe radical atomism, and Stoic ethics required the myth of ekpyrosis. The schools themselves were quite compatible with belief in the traditional gods as were most syncretic Greco-Roman religions. Constantine himself probably didn’t grasp the exclusivity of Christianity (Barnes makes some very good arguments in Constantine and Eusebius). Christianity eventually got around to providing elaborate rationales for its fantastical claims, but just like the philosophical schools, it took quite a long time to work them all out. It took Christianity a lot longer that it otherwise might have because Christian intellectuals were so ambivalent about their classical heritage. Jerome memorably describes a nightmare in which he was accused of being a Ciceronian rather than a Christian. It took a few centuries of hand-wringing and development of exclusively Christian dialectic for such ratiocination to really take root.
Traditional didn’t need such ratiocination because it was traditional. If ancestors believed it, that was good enough for the Romans. Dangerous innovations need explanation and apology. Of course, when the shoe is on the other foot, we see full-throated rhetorical apologies for traditional religion in the letters of Symmachus.