It has always puzzled me that Christians use 2 Tim 3:16 to say that the Bible as we know it is all holy and inspired. When Paul wrote this he was talking about the 1st 5 books of the bible. The bible as we know it had not been written yet, or put together. He did not say that this letter I am writing is holy and inspired. So If you use 2 Tim 3:16 to support the whole bible being holy and inspired, you are implying something that is not there. Is there some other verse that supports the whole bible being holy and inspired, not just the first of the old testament?
You are correct in saying that most of the New Testament had not been written when Paul wrote this. But it’s clear that he was referring to all of what we call the Scriptures, not just the first 5 books. In Romans 15:4 he said that everything that was written in the past was written to teach us that we through endurance and the comfort of the Scriptures might have hope. And in 1 Cor. 10:11 he said that things that happened in the past were written down as warnings to us on whom the fulfillment of the ages has come. All of the Old Testament had been written at the time of these statements.
Most scholars interpret Paul’s claim that “all scripture is God breathed” in 2 Tim 3:16 as applying to the entire Bible, since he was speaking through the Holy Spirit. God already knew the New Testament would be written and the Holy Spirit would inspire its authors to write only what He intended (John 14:26). The Bible has to be true for all people of all ages, telling us what has happened, what is happening, and what will happen.