Everyman must do his own believing. Believing is to accept or regard something as true, but some people at some point do lose thier stance or conviction on issues that was considered to be the truth, when you accept or believe something or any spoken word, it is considered to be the truth. Try avoiding doubts, though successful people have doubts and fears, don't let it overshadow you. One secret of success is, "if you want to achieve success don't doubt your dreams"(Ryan D'souza). If you are going to doubt anything in life, doubt your own limitations and not your dreams.