What is belief?

Recently I set out on a quest to write an essay that would directly challenge a particular conception of belief. A philosopher, whose work I really do not like, has been my philosophical enemy since early in my university level schooling. I debated with “Christian philosophers” regarding his view that authentic and valid belief is nothing more than an assumption. But then I realized something. My essay would not have much force if I did not have a clue of what belief actually is! What is belief? When do we actually believe something? Not an easy question if you begin to think about it. I know it is not the same as simply assuming. I also know it is something that must be accompanied by reasons. Those reasons must also be rational (logical) and experiential. It is in way related to knowledge. But I have never had a eureka experience with respect to this issue. What is it? Any thoughts?