Why Do We Think the Bible is the Word of God?

Below is an excerpt from Norm Geisler’s Systematic Theology Volume 1 (P. 495):

  That the Bible is the Word of God can be discerned from several biblical affirmations:

(1)     that it is God-breathed;
(2)     that it is a prophetic writing;
(3)     that it has divine authority;
(4)     that it is what God says;
(5)     that it is called “the Word of God” or the like.
The Bible Is God-Breathed
Paul declared that “All Scripture is God-breathed and is useful for teaching, rebuking, correcting and training in righteousness” (2 Tim. 3:16).
This Word, often translated “inspired” (cf. kjv), means to be spirated—breathed—from God. A kindred idea is found in Jesus’ words: “Man does not live on bread alone, but on every word that comes from the mouth of God” (Matt. 4:4).
The Nature of a Prophet
As previously elaborated, the Bible claims to be a prophetic writing (Heb. 1:1; 2 Peter 1:20–21); prophets, as mouthpieces of God, spoke only what God put in their mouths (Deut. 18:18; 2 Sam. 23:2; Isa. 59:21; cf. Deut. 4:2).
The Divine Authority of the Bible
That the Bible is the Word of God can also be determined from the fact that it has divine authority (Matt. 5:17–18); Jesus said it was exalted above all human authority (Matt. 15:3–6).
The Bible Is “What God Says”
Often the words of the authors of Scripture are equated with the words of God. For example, cross-reference Genesis 12:1–3 with Galatians 3:8, and Exodus 9:16 with Romans 9:17—it is verses like these (see chapter 13) that give rise to the statement “What the Bible says, God says.”
The Bible Is Called “The Word of God”
This very phrase or its equivalent is used many times of the Bible in part or as a whole. Second Chronicles 34:14 speaks of “The book of the law of the Lord given by the hand of Moses”; Zechariah 7:12 refers to “The words that the Lord Almighty had sent by His Spirit through the earlier prophets.” (See also Matthew 15:6, John 10:35, Romans 9:6, and Hebrews 4:12.)

Norman L. Geisler, Systematic Theology, Volume One: Introduction, Bible (Minneapolis, MN: Bethany House Publishers, 2002), 495.