I believe that, if I turn the door-knob and pull, the door will swing wide, an opening will appear in the previously-solid wall, and I will be able to walk to the other side.
My belief is not reality, but a model of reality held in my mind; a map full of approximations, errors, and guesses, assembled from my inherited intuitive sense of physics and what I have learned by bumping into and not bumping into things. If my map has a threshold fidelity to reality, then the future I imagine when grasping the knob will come to pass.
The belief-maps I choose to test against the seas of Reality are the ones in which I have the most confidence, an observation that gives rise to my preferred operating definition of a belief: A belief is a model of reality in which we have confidence.
A belief, so defined, may or may not be an accurate model; we simply think it is. Imagine it is. Hope it is. Fear that it is. If the predictions made by our models come true, if our imagined future matches the actual future that results when we act according to our map, we can have increased confidence in the accuracy of our model.