The Humanity of Christ as Instrument of Salvation in the Theology of Thomas Aquinas

Interpreters of Thomas Aquinas’s Christology have long observed the uniqueness of his claim that Christ’s human nature is “an instrument of the divinity.” For Aquinas, the mysteries of Christ’s life, the actions and sufferings he underwent in the flesh (acta et passa Christi in carne), cause our salvation in the present. And they have this effect because Christ’s humanity is a “conjoined instrument” of the divinity, united to the Logos, the second person of the Trinity, in person. By contrast, nearly all of Aquinas’s medieval contemporaries, like Bonaventure (1224-1274) as well as Aquinas himself, argued that the humanity of Christ is the meritorious cause of salvation. On this view, God sets up a set of conditions in which he imparts grace when Christ as a human being meets those conditions. Christ’s human actions only prepare us to receive grace, which God alone imparts as the efficient cause. However, Aquinas alone attributes efficient causality to Christ’s humanity, an instrument of the divinity, in virtue of which Christ’s mysteries save us directly. This dissertation defends Aquinas’s “instrument doctrine” as a true and illuminating account of Christian salvation. Unlike most recent studies of the doctrine, which inspect how Aquinas’s account of it developed over the course of his career, this study expounds the doctrine, defends it from objections, refines it, and characterizes its benefits for Christian theology. Aquinas inhabited the intellectual contours of Greek Christology, specifically of John Damascene and Maximus the Confessor before him, and developed his own account of the instrument doctrine based on their works but expanded it beyond them. However, his point of departure for the doctrine was not primarily the fathers, but the New Testament. Scripture claims that God saves human beings by God’s own actions and sufferings in Christ. The question that Scripture raises, then, is this: how can God use his own actions and sufferings to produce states of affairs that he can only bring about as God? Aquinas’s answer is that Christ’s human nature is an “instrument of divinity”: that the Logos saves us by his instrumental humanity as an efficient cause. However, the instrument doctrine has many problems that Aquinas did not consider, including, for instance, whether a nature can be an instrument of a person extrinsic to that nature. Aquinas’s account of instrumental causality, too, is not entirely convincing. Responding to these objections aided by the works of John Duns Scotus (d. 1308), this study argues that the doctrine deepens our understanding of Christian salvation taught in the New Testament and as Christians experience it as the mystical body of Christ