A start-up that mostly flourished after they were bought out by the enormous Roman Empire.

This is a fantastic analogy.

Christianity: The YouTube of 0AD

There was enough flourishing before to prove the point.

Prove what point? Take off your retrospective determinism glasses for a second and note that for any alien species, there's probably going to be a few religions which have double-digits of the population. These religions started from nothing at some point, so of course they look extremely tenacious in retrospect, when in fact any other equally viral religion could have taken their place if history had played out slightly differently.