Everyone believes in something. Even Atheists. The problem arises when that something, isn’t the same thing as someone else’s something. Worse yet, when those two someones who believe in different things use the same words for them. Differences over the meaning of “god” have caused wars. There have been great upheavals when two powerful people have looked at the same ancient text and decided that it tells them radically different things about how to live.
And these are just examples from real life.
In fiction, you would think that we could come up with some truly spectacular stories about the differences between various sects. We probably could… but for the most part, we don’t.