Why is the Bible God's Word?
John Frame writes:
"How then do we know that Scripture is the Word of God? By its self-witness (normative), by facts and evidences (situational), and by the Spirit's working subjectively in our hearts (existential), enabling us to see the Bible's claims as evidences in their proper light."

Let's make that a bit clearer: Scripture is God's Word objectively, whether one sees it or not. The self-witness of God in his own Words proves to us that He is really speaking. The facts and evidence overwhelming show that no one could have made up this story and had it come out so perfectly, especially considering that it was written by over 40 authors in three languages over a period of 1600 years. Finally, without God's Spirit enabling us to see the beauty of God in the face of Jesus Christ, we are left blind in the face of the bright light of truth and glory. (2 Cor. 4:6)