Recently my friend Josiah Neely mentioned offhand how in The Decline and Fall of the Roman Empire Gibbon argued that one reason paganism couldn’t reverse the tide of Christianity is that once a society or individual became Christian and ceased pagan practice, there wasn’t a good roadmap on how to reembrace the old traditions. In contrast, Christianity’s ideological content meant that even if there was a period of apostasy or public cessation of practice, ideological continuity could be maintained.
To be more explicit and extend the argument, I think the key is that there was a class of religious professionals who were devoted in a deep way to the ideological content of Christianity. If, as occurred in Britain and the Balkans, Christianity collapsed, they would endeavor to reconvert the populace, as they did.
There are other cases of this. Han Yu was a Confucian scholar who denounced Buddhism in the year 800 A.D. during the Tang dynasty. This period, between 600 and 900 A.D. was the “high water” point of public state Buddhism in China. But there always remained an alternative tradition of Chinese scholars and officials who were expositors Confucianism. Eventually, these people “recaptured” China during the Song dynasty for Confucianism, and Buddhism became a religion of the popular people and not the state.
As I said elsewhere, this may explain the persistence of Hinduism. Hinduism, like Greco-Roman paganism, is diverse and variegated. But unlike Greco-Roman paganism, there seems to have been a dynamic and reciprocal tension between philosophy and folk religion, mediated by Brahmins. Greco-Roman paganism was fundamentally an expression of ethnicity and identity. Tradition and custom. Greco-Roman metaphysics was the purview of secular philosophers. Hinduism is arguably more fused, and this may have given it more robustness.