All Scripture is God-breathed and is useful for teaching, rebuking, correcting and training in righteousness (2 Timothy 3:16 NIV)
All does not mean just some, or the parts we like. Paul was obviously specifically referring to the Hebrew Scriptures (the Old Testament), summarized variously by well-known two or three divisions. New Testament writings also began to be recognized early on:
... Paul also according to the wisdom given unto him hath written unto you; As also in all his epistles, speaking in them of these things; in which are some things hard to be understood, which they that are unlearned and unstable wrest, as they do also the other scriptures, unto their own destruction. (2 Peter 3:15–16 KJV)
The core books of the Bible are agreed upon by all Christians. Peter confirmed how God inspired the Scripture to be written.
Above all, you must understand that no prophecy of Scripture came about by the prophet’s own interpretation of things. For prophecy never had its origin in the human will, but prophets, though human, spoke from God as they were carried along by the Holy Spirit. (2 Peter 1:20-21 NIV)
This counteracts the lie that the Old Testament was just myth and legend, made up by superstitious people. This infers the submissive will, personality and cooperation of the writer. As such, it is perfectly reasonable to call the Bible the Word of God.
