The doctrine of sola scriptura (or “the Bible alone”) is the sole and only authority in Protestantism. Where is this foundation of all of the Protestant beliefs given to us by a revelation from God? How do we infallibly know that this doctrine is true? If it is true, where in the Bible is it taught, and where was it taught by the apostles?
This is an excellent question. Second Timothy 3:16 focuses on the fact that all the Old Testament scriptures are inspired by God and thus are truthful. There was, of course, not yet a New Testament canon during the NT era itself; the books were still being written. But the principles of holding a very high view of scripture, and basing your life on God’s word as the final authority, are clearly present in texts like Psalms 119. It is because of such texts, and also because of the ecumenical church counsels in the fourth century A.D.–which said these 27 books are our NT canon–that the Protestant churches have held up the notion of sola scriptura: of scripture alone being the final arbiter of truth.
—Ben Witherington III