The Old Testament was written from roughly 1400 to 430 BC and you can see within the documents themselves God giving guidelines for whether prophetic words were true or really from him. For example, in Deuteronomy 18 and it seems there’s this progressive recognition through time of the prophets truly speaking for God and their their writings being preserved. It’s clear when you get to the time of the New Testament, the way that Jesus and the apostles referred to the Old Testament, the way it’s quoted, the way those quotes are introduced, there’s a recognition that these writings are closed, this Old Testament prophetic canon is not continuing to be written and these things are authoritative. As Jesus says in John 10, the scripture cannot be broken.
Josephus, who wrote in the first century, a Jewish historian, said the Jewish canon had been completed and closed since the time of the Persian king Artaxerxes, which was in the 400s. And so it’s pretty easy for Christians to affirm the Old Testament canon. We say we believe the Old Testament canon that Jesus and the apostles believed, very simplistically. So if Jesus and the apostles affirm this collection of writings, that’s sufficient for me. And in fact if you go to a Jewish synagogue today, the Hebrew scriptures that you find there are exactly the same books that we have in our 39-book Old Testament Protestant canon. They’re arranged in slightly different order, they’re grouped differently but it’s the same content in the Hebrew canon in the Jewish synagogue and in the Protestant evangelical 39 book Old Testament canon.