This article is also know as World Building 1A.
I personally always recomend creating your own game worlds for your campaigns. It makes the game your own, rather than something you are just using. Most people will not run characters that other people create. why should game worlds be any different.
While we’re talking about the subject of constructing religions, I’d like to bring up the concept of the Creed. The creed is nothing more than a simple statement of the religion’s beliefs, meant to be recited. If you cannot write a creed for your monotheistic religion, it probably needs work on internal consistency. Creed examples:
Do you ever give ice ages a thought?
Game environments are not built with a ruler and some tape, they are built with imagination and an understanding of what is needed to make the game environment. If you take the time to learn about what you will need to do before you being the process, it will make for better results in less time with less work.
A guide on creating legal codes for games, history, examples, and why bother?