In Summa Theologiae III.42.4 Aquinas asks whether or not Christ should have committed his teaching to writing, and in this article I offer an in-depth analysis of his answer. I begin with a point-by-point account of III.41.4, noting that Aquinas's concerns are with salvation-history rather than with questions of the practicalities of preaching and theologizing or with problems of hermeneutics. Drawing on other parts of the Summa, I then examine the three principal reasons why, according to Aquinas, Jesus should not have committed his teaching to writing – his excellence as a teacher, the excellence of his teaching, and the requirement that his teaching should be disseminated in an orderly manner. Observing that for Aquinas ‘Christ's action is our instruction’, I show how his actio operates as both mystery and teaching to implant the New Law within human hearts. Jeremiah 31:31–33 and Hebrews 8:8,10 are key texts for Aquinas, who believes that the promise that the New Law would be written on hearts could never have been fulfilled had Christ committed his teaching to writing.
