II TIMOTHY 3:16-17 … WRITING BREATH
"All Scripture is breathed out by God and profitable for teaching, for reproof, for correction and for training in righteousness, that the man of God may be competent, equipped for every good work."
This passage comes from a letter of admonition Paul wrote to Timothy, the young minister in Ephesus. At the time of the writing there was obviously no New Testament or Bible incorporating what we know now as the Old and New Testaments. However, there was the Old Testament in the form of the Tanakh, or Hebrew Scripture.
So, want a weird loop? Paul was writing about the Scriptures being inspired by God while writing Scripture that was inspired by God.
The idea that God would use people to write His inspired word was nothing new. The Law and the Prophets all were inspired by God. How is this known? Well, points could be made on the fulfillment of the majority of the prophecies delivered in the Hebrew Scripture. Furthermore, an argument could be made about the supernatural ability of the Law which governed Israel, and eventually the rest of the world, to prevent humankind from destroying itself. This same Law was both inspired by God and included in Old Testament Scripture.
But ultimately, the understanding that Scripture is God breathed is dependent upon faith. If that faith does not exist then no argument or valid point will matter and the one receiving Scripture will be without profit. But if that faith does exist, indeed, as God tells us through Paul’s inspired writing, all Scripture will be useful in completely preparing any person for every good work.