You may straightjacket your creativity. This is a major problem in religious design - you need to pass the point of knowing about existing religions to the point where you can create your own. It's not as difficult as it sounds, but at the same time, it usually doesn't come without some kind of study or knowledge. However, in designing a religion, I've found a few things to keep in mind that help a great deal. Think about these when designing and studying, and they'll make your work go faster.
Religion happens for a reason.
Actually, as I've noted many times in this column, everything happens for a reason, and a culture's traditions and religion are no exception. Also ask what will not be within the sphere of a culture's religion - if anything. Religions change and evolve Religion changes, it adds more ideas, loses others, faces new challenges, and deals with new technologies. Whatever eternal truths there may be, their trappings are not always the same. Some of the Old Testament is nothing more than a record of people facing challenges and dealing with them, and finding new insights and new ways to act.