One problem for the modern reader of Aquinas is that he expected his students to make connections between the different parts of his doctrine for themselves, and in that way approach theological understanding. While this freed the medieval master of needless repetition in his teaching, we today somehow stand in need of modern masters to point out the connections for us, so that we can be freed of the risk of failing to appreciate the richness of his theological vision. This important work, largely begun by Gilles Emery OP, especially regarding the Trinity and creation, has now been skilfully and penetratingly extended by his student Dominic Legge OP to illumine the connections between Aquinas's Trinitarian doctrine and his Christology. If anyone suspected that Aquinas's theology of the incarnation took scant account of the fact that it is the Second Person of the Trinity who takes flesh, or that the Holy Spirit had little part to play in Aquinas's elaboration of his doctrine of the Word Incarnate, then Legge is the contemporary master who provides a full and definitive answer to such misapprehensions.
The book falls into three parts. Part I concisely expounds Aquinas's Trinitarian doctrine and its relation to the economy of salvation, showing the eternal inseparability of the persons in their mutual relations, and the link between the eternal processions and the return of creatures to God through the temporal missions of Son and Spirit. Legge is thus well placed to establish that, for Aquinas, the One who is incarnate is very much the Word of the Father (Part II) and the Word breathing forth Love (Part III), who by giving us his Spirit leads us to his Father.
Part II judiciously sets the record straight with regard to how Aquinas sees it as most fitting, but not absolutely necessary, that it be the person of the Word who takes flesh. Legge ends Part II with an extremely helpful section on why Aquinas held that any divine person could become incarnate, but the bulk of his material is concerned to elucidate why it should be most fitting for the Second Person, precisely as Word, Son, Image, and Author of Sanctification, to be the incarnate One who conveys us to the Father through a filial mode of being and acting.
In Part III, Legge turns to the work of the Spirit who is given without measure in Christ's soul. He expounds this initially in relation to Christ's fullness of habitual grace, his beatific vision, and his infused knowledge. However, Legge takes the view that the Spirit's grace is present in Christ's soul not merely because he needs it for the work of salvation, but fundamentally because he just is the divine Son from whom the Spirit eternally proceeds. In other words, it is as though God could not cause an incarnation of the Son without his soul being (temporally) graced by the Spirit's invisible mission. Here Legge distances himself from Jean‐Pierre Torrell OP's account, where habitual grace is most fitting to the hypostatic union but not absolutely necessary (p. 143). Tellingly, Legge proposes no alternative interpretation of the text on which Torrell relies. While the texts Legge presents certainly show that habitual grace follows in order from the hypostatic union as a matter of fact, it seems to me that they do not quite demonstrate that Aquinas held that this grace had to follow by absolute necessity.
Legge is right to say that Aquinas taught that the invisible missions of Son and Spirit are inseparable, including to the soul of Christ, but when he concludes that the visible mission of the Son and the invisible mission of the Spirit must likewise be inseparable, there is a gap in his argument (p. 150). Aquinas bases the inseparability of the two invisible missions not just on the eternal inseparability of the divine persons, but on the fact that these missions share one of their constitutive temporal elements in common, and this he needed to do because divine missions are constituted precisely by adding temporal effects to the eternal processions. Thus Aquinas argues from the fact that both invisible missions take place by sanctifying grace to their inseparability (Summa Theologiae, I, q. 43, a. 5 ad 3).
Hence, what Legge may need to do in order to establish the inseparability of the Son's visible mission and the Spirit's invisible mission, and thereby the absolute necessity of Christ's grace, is to identify a constitutive temporal element shared by these particular missions. Sanctifying grace would surely fail as a candidate, because the hypostatic union is not constituted through this grace, which rather follows the union, as Legge recognises. Until this gap in the argument can be filled, it seems to me that Torrell's account should be preferred.
Legge is at his best in his final chapters, which explore how Aquinas places the Spirit at the centre of his account of Christ's human action, and deploys instrumental causality to move beyond the Augustinian consensus that Christ gives the Spirit as God and receives him as man to the claim that he also gives the Spirit by way of his humanity. Legge emphasises the fact that Aquinas sees all of Christ's human actions as theandric and salvific, even the most quotidian. However, I do wish he had commented on the possible implications for Aquinas's theology of the incarnation of the two quotations in which Aquinas envisages even the angels’ grace as causally dependent on the grace in Christ's soul (p. 220). Nevertheless, what Legge achieves in the final part of this book is far more than the conclusion that Aquinas does not neglect a place for the Spirit in his Christology. Rather, he convincingly demonstrates that Aquinas's position successfully expresses the co‐ordinated roles of both Word and Spirit in the perfection of Christ's humanity and his saving acts.
