Are you living your life as if that were true? Does your belief in God affect the way you live, or do you still do entirely as you please?
If you say that you believe in God, then how do you know what He wants for you? Are you in contact with Him? Do you just trust whatever you feel is right?
The Bible says that the Devil believes in God (James 2:19), and he is lost. If all you do is simply believe that God exists, then you are no better off than he is. It is not intellectual acknowledgment of God's existence that God wants but your accepting the sacrifice Jesus made on behalf of sinners that pleases God. Simply believing is not enough. You must choose to follow Him.
It is not that you believe. It is whom you put your faith in. Who is this God you believe in? Is He the Christian one? Is he Allah? Is he from another planet? Is he whatever you feel is right? Is he loving? Believing in God is fine unless your god is false. The important thing is that you must believe in the true God, not a false one, and the true God is found in the Bible.