God exists. He exists as He is revealed by the Bible. The reason one must believe that He exists is because He said that He exists. His existence must not be accepted on the basis of human reason, because that is limited to time and space and has been corrupted by indwelling sin. God has sufficiently revealed Himself in the Bible, but He has not revealed Himself exhaustively. Man can know only what God has revealed in Scripture about His nature and works. But that is sufficient for people to know Him in a personal, saving relationship.