The conclusion recapitulates the argument of the book and lays out some guidelines for further research into the instrument doctrine and related theological topics.

The argument of this chapter is that the instrument doctrine, if held, sharpens our understanding of other mysteries of the Christian faith, including the hypostatic union, the Church, and the sacrament of the Eucharist. One reason that Aquinas held to the doctrine was the clarity it brought to these other mysteries. With Christology, the benefits are grammatical and speculative. Calling Christ’s humanity an instrument leads us to think of the one who possesses it, namely, the Logos, and so it can render single-subject Christology clearer to the intellect. If Christ’s humanity is held to be an instrument, it also implicates in our understanding the divine power through which the Logos works. With the theology of the Church, the instrument doctrine supports the biblical idea that Christ sends the Holy Spirit on the Church as a human being, and not only as God. And with the theology of the Eucharist, the instrument doctrine assists Catholic faith in perceiving how Jesus gives us a share of the divine life through sacramental communion.

Chapter 1 introduces the instrument doctrine in Aquinas’s thought and explores its foundations in Scripture, focusing on Aquinas’s biblical commentaries. In his commentaries on Romans and 1 Corinthians, among others, Aquinas argues that the logic of scriptural teaching suggests that Christs’ humanity causes divine effects as instrument of the divinity, including our resurrection. The chapter shows how Aquinas interpreted the Scriptures as coherent with the Catholic tradition, especially the conciliar teaching on Christ in the early ecumenical councils. Aquinas thinks that the doctrine should be understood within the conceptual matrix of these early councils’ teaching on Christ.

The introduction states the biblical premise of the book’s argument. In Scripture, God saves human beings through the actions and sufferings of Christ in the flesh. St. Thomas Aquinas developed a theological account of the Incarnation that attempts to account for the way Scripture speaks, namely, that Christ’s humanity is the instrumental cause of salvation, or as the book calls it, "the instrument doctrine." The introduction then gives an overview of the book’s argument: this doctrine best accounts for how Jesus Christ saves Christians in virtue of his humanity. It outlines the argument of the seven following chapters.

This chapter poses the most difficult objection for the instrument doctrine, in particular as Aquinas conceives of it. For Aquinas, a created cause, Christ’s humanity, produces divine effects as an instrumental cause. But the tradition has affirmed that God alone is the cause of grace in the soul, and no created cause can produce grace. John Duns Scotus puts this objection to Aquinas’s account of instrumental causality, and this chapter argues that the criticism appears to succeed. If a created cause participates in the production of grace, as Aquinas argues, then Scotus argues that Aquinas fails to maintain the distinction of natures and powers in Christ basic to Chalcedonian Christology. For Christ’s humanity is taken up into God’s power and brings about the deification of the human person immediately, something only divine power can do. The ground is prepared for a response to this objection in the following chapter.

This chapter is the heart of the book’s analysis of St. Thomas Aquinas’s teaching on Christ’s humanity as the instrument of the divinity. It explores the various details of Aquinas’s account, outlining it in five synthetic propositions. These propositions, taken together, form the instrument doctrine as St. Thomas conceives of it. Various ambiguities in Aquinas’s account are presented for consideration, and the chapter makes some judgments about how best to understand Aquinas in his mature works. The chapter concludes with a section on the relationship of language to reality in Christology and why reduplicative propositions, used in a standard mode of theological analysis in the thirteenth century, can clarify how to understand the instrument doctrine.