J. David Moser’s book is aptly subtitled ‘A Study in Thomistic Christology’ because Moser’s aim is not primarily to present a historical account of Thomas’ teaching on the instrumentality of Christ’s humanity but instead to offer a robust articulation and defense of the instrument doctrine for which Thomas remains the standard bearer. Thus, Moser is not satisfied with explicating Thomas’ teaching but seeks to manifest its roots in Scripture and the fathers, and to bolster Thomas’ account with the aid of Scotus and Scheeben. Although not a historical study, Moser’s book is nonetheless historically informative. It is compelling as a work in systematic theology because Moser weaves what he thinks can strengthen Thomas’ account together with Thomas’ own teaching into a coherent formulation of what it means to call Christ’s humanity an instrument of salvation.

On my reading, the book’s most significant systematic contribution is found in Moser’s effort to show that construing Christ’s humanity as instrument helps manifest the intelligibility not only of salvation but also of the incarnation itself. Rightly understood, the instrumentality of Christ’s humanity ensures that there is a single subject in Christ, and it preserves the integrity of his two natures while relating them such that Christ’s operations in each do not merely concur. A theologically fruitful account of Christ’s humanity as instrument requires that the concept of instrument be set upon the proper doctrinal grounding (with Nestorius, for example, providing a contrast case). The instrumentality of Christ’s humanity can manifest the intelligibility of the Church’s christological doctrines so long as the latter provides the norm for how the concept of instrument is applied to Christ’s humanity.

The book is divided into three parts, with the first dedicated to Thomas’ scriptural and patristic sources and to Thomas’ own account. In Chapter 1, guided by Thomas’ biblical commentaries and treatment of the life of Christ in the Summa Theologiae, Moser argues that the instrumentality of Christ’s humanity is implicit in Scripture. Moser’s second chapter, on the Greek fathers, touches on the application of the term organon to Christ’s humanity in Origen, Athanasius, and Cyril of Alexandria before turning to more extended discussion of Maximus and John Damascene. Although Maximus rejected treating Christ’s humanity as organ of his divinity, Moser argues that Damascene’s organ account ‘is grounded in Maximus’ Christology’ (p. 40) and is subsequently taken up by Thomas in its essentials. The third chapter distills Thomas’ mature understanding of the instrumentality of Christ’s humanity, including in its relations to the meritorious and exemplary causality of Christ’s paschal mystery and to the communication of idioms.

The book’s second part is concerned with addressing objections and refining Thomas’ account. Chapter 4 deals with two objections: first, that a human nature cannot be an instrument, since ‘among human beings, person and nature are really the same’ (p. 125), and, second, that instrumental efficiency, as applied to Christ’s humanity, is no different from secondary causality. In response to the first, Moser draws on Scotus to argue that Christ’s human nature does not ‘have proper human personality’ not because Christ’s humanity differs from ours but rather because Christ’s human nature depends on a divine person ‘extrinsic’ to that nature (pp. 130–136). To the second objection, Moser claims that the hypostatic union is the key to differentiating the operation of Christ’s humanity as instrument from the secondary causality of creatures. The objection entertained in Chapter 5 is posed with Scotus: since a divine effect (salvation, grace) can only be brought about by a divine power, to claim that Christ’s humanity causes a divine effect in its instrumental operation entails a confusion of his humanity and divinity. After entertaining possible responses in Chapter 6, Moser invokes Scheeben to the effect that the graced elevation of Christ’s humanity ‘is “extrinsic”, in that the elevation is based only on the instrument’s association with the principal agent, not on any infusion of a power that would violate the distinction of natures and powers in Christ’ (p. 193). The final part and chapter of the book ties the instrumentality of Christ’s humanity to the church as Christ’s mystical body and to the Eucharist.

Although Moser does entertain what he takes to be the best objections to Thomas’ instrument doctrine to advance the strongest formulation of that doctrine, some readers might desire more attention to modern objections to the concept of instrument. Moser touches on Karl Rahner’s and Bruce McCormack’s objections in his introduction and conclusion, and he seems to have them implicitly in mind elsewhere, but they are not foregrounded, presumably because Moser deems their objections more tractable than those he deals with at length. Thomist readers might also desire engagement with the ‘one- or two-esse Christology problem’ that Moser claims is outside the scope of his study (p. 141n39). No study can cover everything, but one could reasonably contend that this particular problem has more to do with the question of why Christ’s human nature does not have proper personality than Moser indicates. Then again, Moser may be tipping his hand in employing Scotus as a complement to Thomas in addressing that question.

Moser’s book offers a compelling case that the instrument doctrine Thomas developed helps to manifest the intelligibility of the incarnation and salvation in Christ. The book is a carefully undertaken study with much of value for theologians from a variety of specialities.