None! The Bible simply assumes the existence of God. As has been previously noted (Heb 11:6) God desires that people come to him by faith. This tremendous truth is brought out in John 20:29; 2 Corinthians 5:7; and 1 Peter 1:8.
Clark Pinnock describes how this implicit assumption appears from our vantage point: For the Scripture then, the existence of God is both a historical truth (God acted into history), and an existential truth (God reveals himself to every soul). His existence is both objectively and subjectively evident. It is necessary logically because our assumption of order, design, and rationality rests upon it. It is necessary morally because there is no explanation for the shape of morality apart from it. . . . It is necessary personally because the exhaustion of all material possibilities still cannot give satisfaction to the heart. The deepest proof for God’s existence apart from history is just life itself. God has created man in His image, and man cannot elude the implications of this fact. Everywhere their identity pursues them. (Set Forth Your Case, p. 77)