You're asking questions for which there are few answers. To answer adequately, the responder must believe also in one God, and that's an assumption. I think the easiest answer is that not everyone does believe in one God or they believe their manifestation of God is better than others. "Our god is better than your god" has been the theme song to countless religious wars, though to be thorough, religion is often - way too often - simply the veil behind which lies power, status, wealth, political ideology and the need to subjugate other men.
