All Scripture is inspired by God. Two key verses which speak to this are 2 Peter 1:20-21 and 2 Timothy 3:16. Peter says that no prophecy of Scripture is a matter of one’s own private interpretation. The Prophet in the OT engaged in preaching and prediction. The word prophecy simply means “divine revelation” and can be applied to the Bible as a whole – that is the way that Peter uses the word here. Then he goes on to say in (v. 21) that human beings did not decide to write Scripture, but rather were “moved along” by the Holy Spirit. The writers were fully conscious and used all their faculties and emotions and will. Along with being “moved along” by the Holy Spirit, they had their individual purposes in writing, and were not motivated by the same purposes.
Then Paul writes in 2 Tim 3:16 and says “ALL Scripture (pasa graphe) IS inspired.” This verse receives even more strength its context. Paul writes in regards to the difficult days that lied ahead for the church and is saying in light of that – the Scriptures are God speaking to you. However at the time of writing the NT canon was not in place. Evidently he is speaking specifically of the OT. “Closing the argument in this way allows Paul to bring the discussion full circle back to Timothy and ministry…specifically related to the use and usefulness of Scripture…they were Timothy paradigm (Towner 594).” Moses (Deut 18:18), David (2 Sam 23:2), Isaiah (59:21), as well as all the human writers (Zech 7:12; Ps 110:1 etc) all claimed inspiration in their writings.
Concerning the NT Paul says in (Gal 1:11-12) that he didn’t received the gospel and the message of God from any human source. In the OT tradition was handed down generation to generation. That is not how Paul got it. He received it directly from the risen Lord Jesus on Damascus road. Ephesians 3 he says that the mystery had been given to him to deliver.
The Bible is divine in origin – the ultimate source being God Himself. “The authority of Scripture means that all the words in Scripture are God’s words in such a way that to disbelieve or disobey any word of Scripture is to disobey or disbelieve God (Grudem 73).” It is unfortunate that even those who stand in pulpits in our land do not believe the Bible is inspired by God – every jot and title. They have no power nor source of truth for anything they do. In this culture of relativism it get worse by the day. People today say that there is no source for absolutes. The Bible is alive and powerful (Heb 4:12); it can bring a dead man to life (spiritually).
That being said there is definitely a degree of mystery surrounding the inspiration of Scripture. “Regardless of this…God was able to make His word certain without destroying the freedom and personality of the authors. The human authors of Scripture were not mere secretaries taking dictation; their freedom was not suspended, and they were not autonomous. What they wrote is what they desired to write in the style they were accustomed. God in His providence engaged in a divine concurrence between their words and His so that what they said – He said (Geisler 238).”
Towner, Phillip; NICNT – Geisler, Norman; Systematic Theology – Grudem, Wayne Systemtic Theology