…Christianity did more than end the cognitive decline of Imperial Rome. It reversed that decline and launched an upward trajectory that would last throughout the Christian Era from Late Antiquity to post-medieval times. Thus, with each generation, the average person became more and more intelligent.
The new faith restarted cognitive evolution by banning polygyny, which had usually involved elite men and lowborn women. Elite women thus gained in reproductive importance. The Church thereby changed the culture not so much by making radically new rules but rather by reviving old rules and enforcing them more effectively.
Keep in mind that Christianity was much more rules-based than the old faith, particularly the watered-down paganism of the Imperial Era. Pagan Romans were more interested in developing a transactional relationship with their favorite god: “do this for me, and I’ll do that in exchange.”
How Christianity rebooted cognitive evolution
American Buddhist Net 