Hm, question with assumptions much?
- The Roman Empire was not a Gene Rodenberry Humanist utopia. It was fertile ground for all manner of (as the Roman sceptics would have put it) strange cults from the East, and it certainly had not turned away from religion in any meaningful sense.
- Lucian derided many of these cults in his Passing of Peregrinus and Alexander the False Prophet (including a very early and somewhat confused mention of Christians) ; that tells you that they prospered.
- Lucian was in the elite, and his scepticism was very much an exception. (He made Greek mythology sound absurd, too; and it’s very hard for us to recover the sense of awe that the myths originally had.) Christianity, like other new religions, took root among the disenfranchised Greek-speaking masses.
- As religions went, Christianity was not more absurd-looking than whatever else was on offer in the Empire. It was absurd-looking from the perspective of post-Temple Judaism; but it has been argued that what made it absurd to Jews is what made it plausible to Gentiles (divine birth, relaxation of Mosaic law).