In my games Buddism and Hinduism are each founded on one continent, and then one continent will found every other religion. So one continent (usually mine) will have 6 holy cities and 3 or 4 different state religions, and thus complicated diplomacy, while the other continent will be slightly behind in tech but of unified religion (all Buddist or Hindu). The Civilization that founds Taoism or Islam converts if they have no holy city already, but I've never seen those religions spread.