(Gamespot)"How does religion work, exactly? Do you have to research an individual religion before you can convert to it, much like you had to research different government types in earlier Civ games? And once you have religion, how do you defend against encroaching religions so they don't convert your cities?"
(Barry Caudill, Civ IV's senior producer) "The first person to discover certain technologies will find the religion associated with that technology, so you don't have to have the technology to convert. The easiest way to keep other people's missionaries out of your territory is by eliminating any open border treaties you might have. That would mean that the missionary unit could not cross your border without its owner declaring war first. Keeping your religion strong by aggressively building religious buildings and being aggressive in pushing your own religion with missionaries will also help."