Written Word of God
Chapter 25 - God's Written Words
Just as the prophets spoke God's Word with the same level of authority as God's voice at Sinai, so His written Word has the same level of authority as His audible voice or His prophets. The revealed Word is meant to be permanent, not momentary - passed down to the next generation - 1 Cor. 15:3; 1 Tim. 6:20.
The Old Testament
The OT asserts and assumes throughout that it is the written Word of God. This idea didn't begin in the 1900's with fundamentalism! Ex. 31:18; Josh. 1:8; Deut. 31:26-28. This culminates in Psalm 119, where God's words are an object of praise and a means of carrying on a personal relationship with God. Jesus quotes the OT as authoritative, says it can't be broken, and calls a Psalm "your law," (Ps 82; John 10:34; Matt 5:17-19). The apostles had the same view. When they quote it, you often can't tell if they mean God or Scripture ("it says" - who? God and Scripture are interchangeable). Paul tells Timothy it is breathed out by God, which means it is His speech. 1 Peter 1:20-21 means that the origin of Scripture is God, not the human authors.
The New Testament
No text comes out and asserts directly that it is God's word. But it is required reading for the church (Col 4:16), must be obeyed as from an apostle who is from God (2 Thess 3:14-15; 1 Cor 14:37-38), and quotes other parts of the NT as Scripture like the OT (1 Tim 5:17-18 quotes Luke 10:7; 2 Pet 3:15-16 quotes Paul).
What writings are God's words? This is the question of canon. As early as 170 A.D. there were lists with just a few books missing from the current list. Early documents that were excluded have gotten a lot of press lately (Gospel of Thomas, e.g.), but it's simple to argue that they were regarded by the church as heretical and thus excluded. The ultimate assurance and criteria of the NT canon is supernatural: God testifies to the church that the texts are indeed His Word. We do not rely on the church alone for this assurance. Some objective criteria we apply are that the texts must be apostolic, ancient, used in worship, and orthodox. The canon is closed, since God has spoken ultimately in Christ. Though He added to Scripture Himself through the centuries, He won't anymore because Christ has finally spoken.