“The divine wisdom of Christ’s teaching also guides Aquinas’s response to the problem of why Christ did not write down his teaching in a book. Christ’s teaching was not meant to be merely understood intellectually; rather, it was meant to be ‘imprinted on the hearts of His hearers’ and to become an interior law. Christ’s teaching was so personally powerful that writing it down could only dilute its force. In the same vein, the realities taught by Christ were too profound to be fully expressed in writing. Aquinas cites Jn 21:25 to the effect that Christ’s teaching is inexhaustibly rich and will never be plumbed by mere human books. Had Christ written a book, ‘men would have had no deeper thought of His doctrine than that which appears on the surface of the writing.’ The written word simply cannot contain all that Christ taught. Finally, Christ’s choice not to write a book fits with his mission of forming a Church. He did not wish to teach people directly through his won written word. Rather, he willed to teach people through the testimony if his apostles and their successors. In this way, a fitting order is preserved: Christ is seen to be the highest teacher, rather than having his writings put on par with those of his followers. On the other hand, Aquinas affirms Augustine’s statement that Christ can be said to have written and spoken through his apostles, since they are his ‘members.’ The New Testament is, in this sense, written under the direct guidance of Christ and reveals to us what Christ wished us to know about his deeds and words during his earthly ministry.”
-Matthew Levering in Christ's Fulfillment of Torah and Temple: Salvation According to Thomas Aquinas.