I think every religion grows on a predictable curve, on this curve there is a point at which its contribution to society peaks relative to its financial assets.
New religions have to integrate existing beliefs in order to spread their popularity. Once they gain enough money and people, however, the marginal cost of altering their doctrines to accept new people rises above the marginal cost of enforcing their beliefs with violence and the threat of violence.
Christianity is the posterchild for this: proto christian beliefs simply involved integrating Theological thinking into pagan practice. As it spread further, co-opting became the main mode of attracting converts: gods became saints and holidays took on secondary meanings. Once it became the principle influence in Europe, this stopped, and converts in the new world were converted forcefully by conquistadors, rather than having their native practice treated with respect, as was done to the early pagan adoptors in Greece and the rest of Europe.
For this reason I prefer smaller religions, where people are still willing to question their shared beliefs if it promotes peaceful and effective relations with humanity at large. Once they reach a certain size, no religion is organized enough to change their doctrines, and the road to war is short.