Christians believe because they are uninformed and encouraged NOT to explore or question their faith.  Jesus was believed to have said not to doubt him, or you will go to hell.  It seems as though Christianity does not sell well with the educated, but back then no one else could read and write but the few who passed on the teachings of the early Church from generation to generation.  Simplistically, most people who believe in God do so because of the many instabilities in their life and the necessity to latch on to something, even if mythical, that comforts them.  Too bad they can't find that trust within themselves, instead of a conflicting cartoon character in the sky.