Belief is mental acceptance of some idea as being true. You accept ideas from others because they are authority figures. This setting of a belief in your mind (usually at a young and trusting age) comes about because you have absolute trust in the authority (generally the parent, sometimes the religious or educational institution, sometimes another trusted outside agency such as a relative, peer, or the media).
This acceptance can come about even when there are facts that contradict it. Reinforcement of beliefs strengthens until, faulty or true, they become a fundamental part of your thought processes. To entrench matters more, now beliefs are tested through the structure of the faulty belief, thereby compounding the problem. What this means is that you only accept information that reinforces the belief. Information that contradicts the belief is rejected.