Looking at the most successful religions, a few candidates spring to mind: * Proselytism: if you don't go looking for converts, you probably won't get them. In

Looking at the most successful religions, a few candidates spring to mind: * Proselytism: if you don't go looking for converts, you probably won't get them. In particular, a lot of religions are inextricably linked with ethnicities; they tend to stay small. * Syncretism: your religion can get a lot of followers if it co-opts existing beliefs, and doesn't try to immediately displace them. For example, look at how Taoism fits in neatly with a lot of folk religion that predates its arrival in the area. Similar examples can be found for most (all?) major religions. * Political fortune. Catholicism, for example, happened to spread through the world alongside European colonialism, especially by Spain. * Luck. There are an awful lot of dead religions that used to be pretty successful. There are probably others, but those are the most obvious ones.