User blog comment:LurkerLordB/Religion in a world without Christianity/@comment-8815453-20120424042016

If both those two major religions haven't existed, I would guess that regional Paganism (Finnish, Estonian, Norse, etc.) would be completely dominant in Europe. Meanwhile in the New World, the Aztec, Maya, and Native Religions would still be dominant, though confined in their own area, and maybe, just maybe the colonisers would adopt their beliefs (not really likely, maybe possible, in the case for Bolivia). As for Asia, I would think (more likely) Buddhism, Hinduism, Taoism, and Shinto would probably have a chance to gain more influence in the world and expand. :]

As for Arabia, they would have probably be dominated by Zoroastrianism, assuming the Persian Empire would have expanded and happened as OTL. Maybe OTL Greece and Turkey would have it too, but maybe in small numbers though.