The Catholic Church and nearly all Christian communions claim that Jesus of Nazareth is fully God and fully a human being, that he came down from heaven, suffered, died, was raised from the dead, and ascended into heaven for our salvation. The Incarnation is the heart of what Christians believe that God has done to save them from sin. Central to the mystery of the Incarnation of the Logos is his suffering and death upon a Roman cross and his resurrection three days later. These events he suffered in the flesh bring about a divine gift, the vision of God, to all who are united to Christ through faith. Through his death and resurrection, we may receive the everlasting “river of the water of life,” which the body of Christ will inherit at the end of time (Rev. 22:1).

How does the incarnate Lord offer this divine gift through his flesh, his ignominious suffering on the cross and resurrection? An important and influential theological tradition, beginning with the Fathers and culminating in St. Thomas Aquinas’s teaching, has offered an answer that consists in a causal explanation. According to these venerable teachers, the second person of the Trinity, the Logos, saves human beings by the power of his own flesh and blood because his human nature is the “organ” (Gk. organon) or “instrument” (Lat. instrumentum) of his divinity. According to St. Thomas, this “instrument doctrine” gives a causal account of why Jesus’s flesh and blood brought salvation both in his earthly sojourn millennia ago and in the mystery of the sacramental life of the Church in the present. The doctrine continues to be influential in modern ressourcement theology that draws upon the Fathers and St. Thomas Aquinas.

In many ways, this doctrine goes to the heart of the Christian mystery and confounds the mind. How can the almighty, eternal Son save us by human flesh and blood, even if that flesh and blood is his own instrument united to him in person? It seems impossible that God could do this, because God alone in his own divine power can save. Creaturely causes in no way save because it is beyond their power to do so. As the LORD proclaims to the prophet Isaiah, “I, I am the LORD, and besides me there is no savior” (Is. 43:11). How can a created cause produce salvation in any way, something only God can cause directly by his own divine will and power? Jesus appears to state the impossible when he says in his great Bread of Life discourse in the Gospel of John, “Those who eat my flesh and drink my blood have eternal life, and I will raise them up on the last day” (6:54). Flesh and blood cannot save, even if they are united to a divine person! It is no wonder many of the disciples heard Jesus say this and murmured among themselves, “This teaching is difficult. Who can accept it?” (6:60). Created realities, even those hypostatically united to a divine person, cannot cause salvation in the soul; only the LORD of Israel can do that.

And yet the New Testament teaches, repeatedly, that Jesus’s human actions produce divine effects that touched not only those who were present for them 2,000 years ago but also the faithful in the present. It insists that all of Jesus’s human actions and sufferings bear saving power. His birth from a virgin reveals the love and humility of God (Luke 2:8–20). He took on the burden of the Old Law by being circumcised, an act that freed us from the Law’s burdensome requirements (Gal. 4:4–5, cf. Col. 2:11–22). He was baptized in the Jordan “to fulfill all righteousness” (Matt. 3:15). His voluntary poverty causes our spiritual riches (2 Cor. 8:9). His teachings purify us (John 15:3). When he was tempted by the devil in the desert, he conquered our temptation, for we are strengthened by his ability to “sympathize with our weaknesses” (Heb. 4:15). His passion and death made satisfaction to the Father for sins (Rom. 3:21–26). He merited our salvation, redeemed us from the curse of the law, and opened the way to heaven (Gal. 3:10–14; Phil. 2:9; Heb. 10:19). His blood obtained “eternal redemption” for the Church (Heb. 9:12, cf. Acts 20:28). His burial draws us into him, for through baptism we are buried with him (Rom. 6:4). His resurrection is the cause of our justification and final resurrection (Rom. 4:25; 1 Cor. 15:20–21; Eph. 2:6). His ascension prepared the way for us to reside with him in perpetuity (John 14:2–3). Offering a summary of the Gospel, John the beloved disciple proclaims: “Indeed, God did not send the Son into the world to condemn the world, but to save the world through him” (John 3:17, emphasis mine). The Son, the Logos of the Father who became flesh, saves us by his human actions and sufferings, and this appears to be the mystery stored up for the fullness of time (Eph. 1:9–10, cf. Gal. 4:4). To put the matter presented in Scripture technically, Jesus’s human causal powers bring about our salvation, a divine effect. Many of the Greek Fathers, observing this scriptural idiom, taught that, if this scriptural way of speaking of Christ and salvation is true, then Christ’s humanity is best described as an organ or instrument of his divinity.

This book is an attempt to investigate the ratio of this ancient doctrine with the guidance of St. Thomas Aquinas (1224/5–74), one of the greatest lights of the Christian theological tradition. St. Thomas, seeking to be faithful to Scripture and the Fathers, argued that Jesus’s human powers produce divine effects because his human nature is “an instrument of the divinity” (instrumentum divinitatis). For Aquinas, the “instrument doctrine” accounts for how God saves human beings by God’s own human actions and sufferings in Christ, or as Aquinas often puts it, the “actions and sufferings of Christ in the flesh” (acta et passa Christi in carne).Footnote ¹ On Aquinas’s view, Christ’s human nature is an instrumental cause: a cause that participates in the power of the agent wielding it to bring about our salvation. In this sense, according to Aquinas, the human nature of the Logos participates in his divine power, and by it, he saves all those united to him by the Holy Spirit.

I argue in this book that the instrument doctrine best accounts for how Jesus Christ saves us in virtue of his humanity, in both his life on earth and in the present through the indwelling of the Holy Spirit and the sacramental life of the Church. To this end, I am seeking to understand how Christ’s flesh and blood save in this way. The book is therefore to be understood as an exercise in what St. Anselm called the intellectus fidei, “the understanding of the faith.” It will explore, first, the logic of the Incarnation and its saving effects, with the guidance of St. Thomas and the Fathers before him. Second, it will both consider objections to it and examine its benefits through doctrinal case studies, showing what would be lost if we rejected the instrument doctrine. And in the end, it will hopefully help Christians by God’s grace to understand better what they believe about the Incarnation by faith.Footnote ²

Thomists might wonder why a book like this is needed now, given how much has been written about this feature of Aquinas’s Christology in the past century. First, because the Incarnation and its connection to salvation is a great mystery of the faith, Catholic theologians will be returning to it again and again until our Lord’s coming. As Matthias Joseph Scheeben (1835–88), the great nineteenth-century Catholic dogmatic theologian claims, the mysteries of the Christian faith are luminous and beautiful.Footnote ³ Their beauty draws the intellect to contemplate the truths of divine revelation they contain and to seek their rational luminosity and coherence. When I first began to read Aquinas on Christology during graduate school, I was struck by the power and mystery of the instrument doctrine as he described it after having spent a time reading the Greek Fathers on Christology. I think he understood the marvel of the claim the Christian theological tradition proposed: Christ’s humanity is elevated by God to produce divine effects. At that same time, I observed that Thomistic theologians who had been writing Christologies or historically informed accounts of Aquinas’s Christology took it to be basic that the instrument doctrine was true. But I wanted to understand better how this doctrine could be true, and I believed there was a need to explore it further and offer the strongest possible account of it, not only so that we can know what we need to affirm about the role of the humanity of Jesus in the economy of salvation, but also how we might understand the Lord’s ways better by his grace.

Second, this book differs from most studies of the topic in that it is an explication and defense of the instrument doctrine, not a historical study of the development of Aquinas’s thought. Most of the account I defend is derived from his mature works, and so I consider my proposal in this book to be a contribution to the scholarly project of “Thomistic Christology.”Footnote ⁴ I understand Aquinas to be a great synthesizer of Greek patristic Christology on the instrument doctrine, and he was able to provide the most thorough account of the doctrine that is present in St. John Damascene and others before him.

Thus, this book is a work of Christian dogmatic theology and is not primarily historical. Much of the scriptural and patristic roots of the doctrine have already been presented for study by Theophil Tschipke, a German Dominican priest who published what remains the standard scholarly treatment of the doctrine in the 1940s. It lucidly presents the core ideas of the doctrine with a compendious analysis of texts. It remains indispensable, not only for its exposition of the Fathers and Aquinas but also for how it contrasts Aquinas with other medieval scholastic accounts of the causality of Christ’s humanity antedating him. However, it neither interrogates the coherence of the doctrine in detail nor defends it from objections.Footnote ⁵ Much more can be said about the doctrine’s coherence and truth, and I offer this book to provide that speculative analysis. Furthermore, recent studies tend to look at Aquinas’s instrument doctrine under the aspect of its historical and diachronic development, generally with less attention to the theological warrants and ends that support it.Footnote ⁶ Put differently, these studies are typically uninterested in establishing by argument whether the doctrine is plausible or a good explanation of the saving mystery of the Incarnation.Footnote ⁷ They tend to have broader historical concerns in mind. While I want to rely on the most up-to-date scholarship on Aquinas’s Christology, I am more interested in arguing in favor of the instrument doctrine as a theological account of the Incarnation and its saving effects. For example, where needed, I will describe how Aquinas developed over time on a particular aspect of instrumental causality. But I do it not to resolve these debates but to attain this dogmatic goal.

Third, objections to the instrument doctrine are common in modern theology. Given this reality, a renewed attempt to think hard about it and major objections to it seems warranted. Some Catholic theologians have claimed that “instrumentalization” of Christ’s humanity is problematic for a variety of reasons. For example, Karl Rahner argued that conceiving of Christ’s humanity as “purely instrumental” would entail that it would then be “purely passive” in relation to the Logos.Footnote ⁸ Thomas Joseph White summarizes Rahner’s concerns: “Classical scholastic presentations … are incapable of promoting such a dynamic, historical vision of the humanity of Christ. The reason is that they treat the humanity of Christ as the ‘instrument’ of the divinity … Christ’s personhood is expressed in uniquely ahistorical terms due to the eternity of the Word.”Footnote ⁹ Rahner thinks that the instrument doctrine undermines the historical presentation of Jesus as a changing and dynamic figure that we find in the Gospels. More recently, Bruce L. McCormack, a Protestant theologian, has claimed that the patristic tradition’s attempt to “instrumentalize” Christ’s humanity divorces Jesus’s human life from the Logos, confining Christ’s “loud cries and tears” to the humanity of Jesus, and not to the Logos, who ought to be the proper subject (Heb. 5:7).Footnote ¹⁰ In both of these objections, it appears that if we accept the instrument doctrine, we therefore reject the hypostatic union, or at least a theologically intelligible account of it. I hope that this book will assist contemporary Catholic and Protestant theologians by assuaging these concerns about the doctrine. I will spend most of the time trying to be clear about what the instrument doctrine entails. Thus, the careful reader of this book will discern that their concerns are founded on misunderstandings and that a careful approach to the doctrine with St. Thomas’s guidance will alleviate them.

The best way to make the case for the doctrine is to pose what I take to be the strongest arguments against it and respond to them. For these arguments, I turn to Blessed John Duns Scotus (d. 1308). This move may surprise my Thomist readers. Why would a Thomistic Christology need to respond to arguments by its historic opponent? I engage Scotus at length in this book because I want this book to stand in a long tradition of Thomist theology that sharpens its argument as clearly as possible by engaging with arguably its strongest critic. This approach was standard practice in the Thomist commentary tradition on Aquinas’s Summa theologiae in the generations after Scotus. Scotus is an acute theological thinker, and his contributions to theology are only recently becoming appreciated in English-speaking theological circles.

In particular, I consider Scotus’s objections to Aquinas’s philosophical theory of instrumental causality that supports his theological account of the instrument doctrine. A Scotist would argue that Aquinas’s theory has a serious philosophical problem that, on the face of it, makes it inconsistent with orthodox Christology. For Aquinas claims that the instrument is raised above its natural capacities to produce supernatural effects by the addition of an instrumental power or created form. Scotus responds that, if Aquinas is right about this, then Christ’s humanity brings about what only divine power can do. And if Scotus is right, then Aquinas fails to distinguish the natures and powers of Christ as the faith of Chalcedon (451) requires. I will argue that this argument has merit, and it will require us to state clearly what Aquinas means when he speaks of a created form in Christ’s humanity. I will argue that this form does not enable Christ’s humanity to produce grace on its own but is the effect of divine power invested in Christ’s humanity, which alone produces grace in us through it. I argue that Scotus’s alternative to the instrument doctrine does not do a good job accounting for what scripture and the Fathers teach about the immediacy of the saving power of Christ’s fleshly life, death, and resurrection as Aquinas’s does, and it marks a significant departure from the Greek patristic tradition that Aquinas tried to synthesize. Furthermore, and crucially, I argue that Aquinas’s account can be defended from Scotistic objections with clarity and precision.

I believe that Aquinas, with the Fathers before him, perceived one of the deepest parts of the mystery of the faith in the instrument doctrine. In this book, I hope to pursue what I take Aquinas himself to be pursuing: the clearest understanding of the truth that Christians believe and love. The main argument of this book is that the instrument doctrine remains the best account of the New Testament teaching about the mysteries of the life of Christ and the saving actions of Christ in the present in the Church and the Eucharist.

The book is divided into three parts. Part I, “Foundations and Statements of the Doctrine,” consists of three chapters. Chapter 1 introduces the instrument doctrine and explores how Aquinas developed his account of it from close exegesis of the New Testament in his biblical commentaries. Then it examines Aquinas’s use of conciliar documents in his mature Christology, for these documents refined his understanding of the doctrine in the matrix of orthodox Christological teaching. In this chapter, I argue that Scripture’s way of speaking attributes divine effects to Christ’s human acts. For example, St. Paul claims that Christ “was raised for our justification” (Rom. 4:25). By careful exegetical attention to scriptural words, Aquinas shows us that the text teaches indirectly that Christ’s humanity produced divine effects as an instrument of the divinity.

Chapter 2 discusses the foundations of the instrument doctrine in the Greek Fathers of the Church. This chapter is not as exhaustive as Tschipke’s study, which canvassed both Latin and Greek Fathers. Though we will examine background for the claim in Origen and St. Athanasius of Alexandria, the chapter will focus on major theologians who were directly influential on Aquinas’s account: St. Cyril of Alexandria, St. Maximus the Confessor, and St. John Damascene. I will argue that St. Thomas’s account of the doctrine is fundamentally the same as the ones we find in the Greek Fathers, though Aquinas provides a highly ramified metaphysical account of it that they do not. Aquinas got his account of the doctrine to a great extent from his study of the Damascene’s De fide orthodoxa, Book III. And John had carefully read and internalized many of Maximus the Confessor’s arguments against the monothelites, sometimes quoting whole passages of Maximus’s directly. Maximus’s work formed the foundation for the Damascene’s own, including his version of the instrument doctrine, though Maximus did not explicitly articulate such a doctrine. This chapter will articulate the instrument doctrine as constituted in five synthetic propositions as they appear in the Damascene’s writings. These propositions are similar to the five propositions that constitute the doctrine as it will appear in Aquinas in Chapter 3. I argue that Aquinas held that his instrument doctrine was faithful to a basic desideratum in patristic Christology.

Chapter 3 develops Aquinas’s account of the instrument doctrine from his mature Christology but with occasional attention to his earlier works where pertinent. It contrasts the doctrine with two different accounts of the causality of Christ’s humanity prevalent among the works of Aquinas and his contemporaries: meritorious and exemplar causality. While Aquinas never rejected these models, he believed that they were insufficient explanations of how God saves us in Christ. For Scripture teaches that Christ accomplished our salvation in the diverse human actions that made up his life, and these actions – his mysteries – cause our salvation efficiently in the present. Christ’s humanity is the instrumental cause of salvation, a cause of a state of affairs only God can produce. This claim led Aquinas to a striking conclusion: if Christ’s humanity is the instrument of his divinity, then all that Christ did and suffered had saving power. The chapter expounds Aquinas’s instrument doctrine in five propositions that are basically the same as those we find in St. John Damascene. This chapter is the core of this book, for it sets out the account of the instrument doctrine it defends.

Part II, “Difficulties and Resolutions,” presents major objections to the doctrine and develops an account of the instrument doctrine that resolves them. It also consists of three chapters. Chapters 4 and 5 consider the objections. Chapter 4 deals with two objections to the use of the concept of instrumentality in Christology. The first objection is that a person’s nature cannot be his instrument, for in human beings, persons are really the same as their natures. To say that Christ’s humanity is his instrument, then, would be to split up the person from the nature, and that would imply that Christ in his humanity lacks some feature of being human that we have. Employing the Christology of Scotus, the chapter argues that created personality is an accidental feature of human nature. For that reason, the Logos can assume a really distinct human nature and use it as his instrument, supplying his own personality to it as his own without any loss to the integrity of his human nature. The second objection is that the instrumental causality of Christ’s humanity seems to be the same as secondary causality. If that is the case, then it is unclear what difference instrumental causality makes in Christology. I argue that Christ as a human being can be called a secondary cause in virtue of his humanity, but because all his actions and sufferings were operated instrumentally by the divine power, all that he did and suffered as a secondary cause was also instrumentally effective for salvation.

Chapter 5 argues that Scotus identified a problem of coherence in Aquinas’s account of instrumental causality that affects the instrument doctrine. Aquinas claims that an instrument gains an “instrumental power” in virtue of the principal agent, reducible neither to the principal agent’s proper power nor to the instrument’s proper power. This instrumental power, and the idea of the instrument participating in divine power, appears to violate the distinction of two natures in Christ by eliding the distinction of causal orders that coheres with this distinction. This amounts to a serious objection to Aquinas’s account of instrumental causality, but I argue that the instrument doctrine, where Christ’s humanity contributes immediately to divine effects, not by its own power, but solely by divine power through its efficiency, is still the best way to account for the way Scripture and the tradition speaks about the saving mysteries of Christ’s life.

Chapter 6 offers a Thomistic account of the instrument doctrine that responds to Scotus’s objection discussed in Chapter 5. According to the account I propose, Christ’s human causal powers cooperate with his divine power to bring about the effects that the Logos intends. Christ’s humanity is elevated by the Logos’s divine power extrinsically to bring about divine effects. With Matthias Joseph Scheeben, I argue that nothing substantial changes in Christ’s humanity as the Logos employs it. Rather, divine power raises Christ’s humanity up to make Christ’s human powers cause divine effects in an extrinsic way: from without. The “extrinsic” qualification here is key, for this is what allows us to avoid Scotus’s “Monophysite” criticism of Aquinas. In this model of “extrinsic elevation,” I hold that Christ’s humanity participates in the power of his divinity, like Aquinas does, but I deny that such participation requires that Christ’s humanity is changed in any way. The created form that Aquinas claims is in Christ’s humanity is only the effect, not the reason, that Christ’s humanity causes grace as an instrument of the divinity. That reason is divine power, which alone causes grace. God is free to use created realities to mediate his power in the world while leaving them unchanged in their natures and powers. I argue that this view makes good sense of Aquinas’s texts in his mature theology. For example, I do not think there is any good reason to hold that the “instrumental power,” as Aquinas conceives it, is intrinsic to the instrument, which would then introduce a natural change, since he does not use “intrinsic” or its cognates when he describes it; later commentary developed this distinction to account for Aquinas’s position. On this account of the doctrine, Christ’s human causal powers reach divine effects not because of a created power in Christ’s humanity but because of divine power moving Christ’s humanity. This proposal will become clearer, we will see, by attention to different analogies, especially patristic ones like heated coal and the union of body and soul, for instrumental causation that Aquinas typically uses, like the axe and the craftsman. This reading allows us to hold two claims coherently: God alone is the cause of grace; Christ’s humanity causes grace as instrument. God brings it about that we are saved by Christ’s humanity. Christ’s humanity is an instrumental efficient cause of grace as it is moved by the divine power, but it contributes nothing of its own to the effects. God makes his power present in Christ’s humanity as its mediator, so that when Christ touches the leper, he is healed. The teaching of Aquinas and the Fathers, following Scripture, is that the hypostatic union makes this state of affairs possible. If God really did unite humanity to himself in person, then God can use that humanity to bring about our salvation by it. The grace of the hypostatic union enables Christ’s humanity to be elevated to participate in divine effects.

Part III, “Theological Implications,” which consists of one chapter and the conclusion, lays out the benefits of the instrument doctrine for a dogmatic theology of Christ, the Church, and the sacrament of the Eucharist. Chapter 7 describes three ways that the instrument doctrine enables us to perceive better the connection of the mysteries of the Christian faith. The first section of the chapter covers two that are Christological, and they primarily belong to the order of knowing. First, the instrument doctrine clarifies that the humanity of Jesus belongs to the Logos, the primary agent, and is an independent terminal subject. Second, it orders the relationship of causal powers in the order of knowing: the Logos brings about divine effects through the mediation of his human powers. The second part of the chapter looks at two ways the doctrine illuminates our understanding of, first, the doctrine of the Church as the mystical body of Christ, and second, of the Eucharist. I will argue that that the fitting outworking of the instrument doctrine is Aquinas’s claim that Christ sends the Holy Spirit upon the Church as a human being. It contrasts Aquinas’s claim Christ is the cause of grace in virtue of his instrumental humanity with the common claim that the man Jesus Christ is the source of grace. It argues that the common claim is not enough to capture the fullness of Christ’s work in his mystical body in the present. For if all that Christ did and suffered in his earthly sojourn was saving for us, then it makes sense to suppose that he continues to use his humanity instrumentally to minister to his body in the present by sending the Holy Spirit upon it to console and to advocate for his members. Then I look once again at St. Thomas’s reading of John 6 and his account of the Eucharist. It argues, with help from St. Cyril of Alexandria, that the fullness of Christ’s body, blood, soul, and divinity in the Eucharist is perceived more clearly in theological intelligence if we understand it to be numerically the same soul and body that is the instrument of the divinity of the Logos.

I offer up this book to theologians and scholars of St. Thomas Aquinas’s theology to aid us all in our journey of knowing and loving the Lord better and, hopefully, to provide a fuller account of this doctrine so that we may all see more clearly the mystery of what Aquinas and the Fathers before him saw: Jesus’s utterly human life, every way like ours except for sin (Heb. 4:15), imparts to us eternal beatitude, the river of life that will know no end (Rev. 22:1).