After determining the god this religion follows, world builders should start with history because we can incorporate this in the invention of a religion. Almost every religion pays great attention to its beginnings. This will include historical figures that performed influential deeds, and the symbols that are crucial to it. The world view and place in society is another early subject to flesh out, as are relationships with species and which ones are typically members and/or clergy. How people worship can come next. The details of becoming a priest are an area to save for later unless we need it. Combat is perhaps the last subject to work on unless fighting is a major aspect of the religion, as with a god of war.