Christ set up a new covenant in his blood, and a public act of its remembrance. Since this covenant is God’s definitive presence with his people, it demanded words attributable directly to God. This first belongs to the words instituting the new covenant (“this is my body”), but these words themselves rested on two foundations: (1) the words of the old covenant which they fulfilled; and (2) the Apostolic words spoken from the foundation of the Church of the new covenant. So the words of institution are evidently divine words, but they are the crown of two other foundations, making these foundations evidently divine.
Scriptural inspiration, or divine authorship, therefore make sense and are justified within a liturgical context, and where this context is not presupposed they might make little sense. For example, a young Augustine was convinced that “divine authorship” must be understood in a context where Vergil and Cicero were the height of perfection, and so he immediately concluded that scripture was simpleminded peasant talk. In our own day we might imagine that the height of perfection is explaining the laws of nature, and so we would look at scripture as a simple minded folktale about the nature of the universe. Both judgments involve the ignorantio elenchi of condemning something by an irrelevant standard. Scripture is divinely authored in order to be the public liturgical words of the new covenant, either by anticipation or manifestation, so if you’re not looking for what the new covenant is offering, you’ll either need a new account of why it is inspired, or abandon inspiration altogether.
Again, you might object that a divine being could/would write a better book: with better prose, better and clearer prophesies, better historical attestation (more footnotes, I suppose,) more amazing explanations of natural science, etc. But any text “fails” against a standard it was not written to meet. Perhaps God would have convinced you with better prose, better science, better Old Testament morals or whatever. As God understands things, however, the only point of convincing you would have been to better dispose you to receive the grace of his covenant, but even the disposition to receive grace is itself grace. Seen from this angle, the objection to scripture assumes Christianity is Pelagianism, where God must first persuade those without grace – with impressive prose or science or prophesies or whatever – to dispose them to receive grace. This is not how it works, since even miracles prove just as effective in setting persons against Christ as for him.
Just Thomism 