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As can be observed thus far, Aquinas’ description of the moral and intellectual capacities of human nature suggests a division of agency within the person of Christ. As we will argue below, Aquinas’ concern in this regard is specifically tied to his assertion that the human nature alone functions as salvific mediator between God and humans. Ultimately, this description of Christ’s human agency nicely parallels the Angelic Doctor’s account of divine grace’s relationship to human agency. For Aquinas, divine grace’s chief function is the activation of the human free will. Such activation makes human agent capable of meriting salvation through his works. This relationship between free will and divine grace exists archetypally within the theandric person of Christ. As the Jesuit theologian Paul Crowley comments regarding Aquinas treatment of the subject: “This relation between the two natures in Christ is paralleled in the patterns of nature and grace that can be predicated of all human beings.”
Chief among the ways in which Aquinas’ view of the person of Christ parallels his view of grace and nature is his doctrine of Christ’s mediatorship. In contrast to what we have previously seen in Chemnitz, Aquinas rejects the notion that Christ is mediator according to both natures. Aquinas asserts on the basis of his interpretation of 1 Timothy 2:5 (Vulgate: “unus enim Deus unus et mediator Dei et hominum homo Christus Iesus”) that Christ is mediator only according to his human nature. Such a position was by no means novel within the theological tradition of the High Middle Ages, but was in keeping with mainstream of the Latin Church as represented by earlier figures such as Augustine, Peter Lombard, and Anselm. Such an understanding of Christ’s redemptive agency continued to be promoted at the time of the Reformation by both Ulrich Zwingli (implicit in his concept of alloeosis) and Francisco Stancarus.
This accent on the duality of the person and work of Christ is derived in the western Christological tradition primarily (though not exclusively) from the Tome of Leo (449 A.D.). Leo (the bishop of Rome in the mid-fifth century and first real claimant to the Papal office) penned a letter regarding the Christological controversy that had erupted in the East (first between Nestorius and Cyril, and the later due to the heresy of Eutyches). He took a different tack regarding Christ’s theanthropic agency than Cyril and the later Greek Fathers. Instead of emphasizing the unity of Christ’s personal agency, the western patriarch strongly asserted the duality of his operations to the point of giving the impression that it was possible to divide them into distinctly human and divine acts in the concrete: "For each form does what is proper to it with the co-operation of the other; that is the Word performing what appertains to the Word, and the flesh carrying out what appertains to the flesh. One of them sparkles with miracles, the other succumbs to injuries." Though it is perfectly correct to recognize a duality of activity within the person of Christ in the abstract, it is problematic to do so in the concrete. As the first part of the statement makes clear (“For each form does what is proper to it with the co-operation of the other”), Leo does not intend to divide the person of Christ. This being said, there is no sense for Leo that the two natures operate in and through one another in the concrete. He describes them as two quasi-separate subjects cooperating with one another.
In keeping with this general Latin outlook, for Aquinas Christ’s human nature alone must be designated as the sole mediator, in that it dies in order to atone for sin and therefore acts as eternal high priest: “While it is true that Christ was a priest, not as God, but as man, it is also true that it was one and the same person who was priest and God.”  In this statement, it should be observed that even when Christ is spoken of in the concrete unity of his person, his role as priest and salvific mediator occur apart from his divine personhood. It therefore may be inferred that Christ, even considered in the concrete, acted as priest and atoned primarily as man in union with God. This does not mean (as we shall see below) that Christ’s union with the divine person has no significance for Thomas. Nevertheless, much like for theviator, Christ’s salvific work is primarily conceptualized as grace (i.e. the divine person and created gifts) activating nature (the humanity) in order to achieve its soteriological goal.
Aquinas sets up his discussion of the relationship between Christ’s human will and divine will by reviewing how various casual agents within the temporal world act upon one another. He states that whereas the movement of inanimate objects and sub-rational creatures can be affected without the consent of the will, rational creatures (humans and angels) cannot. They must be given the capacity in themselves in order to cooperate with the movement of the mover who moves them. For this reason, Christ’s human nature, asserts Aquinas, must be thought of as possessing a capacity for self-determination derived from its own nature. Of course, since Christ’s human nature was augmented with grace and subsisted in a divine person, its volition never came into conflict with the divine nature’s will. Beyond this, due to direct vision of the divine essence, Christ’s human nature was confirmed in the good and consequently always freely moved itself toward the good ends to which the divine nature was prompting it. The divine nature may therefore be thought of as moving the will of the human nature indirectly through the means of the graced human nature’s active cooperation. As is clear from the above description, this volitional unity between the divine and human nature in many ways appears to represent more of a pre-established harmony than a description of the agency of a single theandric subject.
Despite this tendency to divide the personal agency of Christ, Aquinas ultimately does agree that it is possible to attribute the meritorious death of Christ to the person of the Logos. The human nature’s salvific actions in atonement are amplified by its unity with the Second Person of the Trinity. Because of his humanity’s unity with an infinite person, Christ’s satisfaction was infinite. Elsewhere, Aquinas writes: “One who was merely a man could not make satisfaction for the entire human race, and how could God? It was fitting then for Jesus Christ to be both God and man.” Nevertheless, although both Chemnitz and Aquinas assert the unity of Christ’s soteriological agency and the infinity of his satisfaction, the emphasis on who the agent of redemption is remains very different. Despite the fact that Aquinas often describes the human nature as the salvific instrument of the divine agent (instrumentum divinitatis), the accent of his teaching falls very heavily on the human nature’s work as a salvific mediator over against the divine person. By contrast, Chemnitz conceptualizes redemption as the work of the divine person acting and present within the humanity.
In describing the subjective unity of the person of Christ, Aquinas’ tendency to assert a strong division between the two natures expresses itself in other ways. For example, when discussing the worship of Christ, Aquinas claims that Christ's flesh can be worshiped in the concrete unity of the Incarnation much like a robe can be honored along with the king who is wearing it. The problem with this analogy is that whereas Christ's humanity subsists in the person of the Logos, a king's robe is an independent entity that does not. Such an analogy suggests a very tenuous unity between the two natures at best. That is to say, the unity between the king and his robe is, after all, merely notional or rhetorical (i.e., it is conceptually present in the mind and social gesture of one who pays obeisance), but not real and hypostatic.
In further discussing the worship of Christ’s human nature, Aquinas goes a step further and asserts that when considered in the abstract, the human nature cannot really be worshiped (latria) at all, but can only be venerated (dualia). That is to say, Christ’s human nature possesses no more exalted status than that of the glorified saints, who also receive dualia. Again, this seems to assume that the human nature possesses a status and reality in itself, semi-independent of the hypostatic union.
 ST 1a2æ. q. 114, art. 2; BF, 30:204-7.
 Paul Crowley S.J., “Instrumentum Divinitatis in Thomas Aquinas: Recovering the Divinity of Christ,” Theological Studies 52 (1991): 454. Aquinas makes this connection himself. See ST 3a. q. 18, art. 1; BF, 50:66.
Ad primum ergo dicendum quod quidquid fuit in humana natura Christi, movebatur nutu divinae voluntatis, non tamen sequitur quod in Christo non fuerit motus voluntatis proprius naturae humanae. Quia etiam aliorum sanctorum piae voluntates moventur secundum voluntatem Dei, quae operatur in eis et velle et perficere, ut dicitur Philipp. II.
 Chemnitz, 81; Preus, 215.
 ST, 3a. q. 26, art. 1; BF, 50:206. See Pesch, 307, 313.
 Augustine, The City of God, 11.2; NPNF, 2:206.
 Lombard, 3:81-4. Also see discussion in Philipp Roseman, Peter Lombard (New York: Oxford University Press, 2004), 118-43.
 Anselm, Cur Deus Homo? 2.28, in A Scholastic Miscellany: Anselm to Ockham, trans. and ed. Eugene Fairweather (Philadelphia: The Westminster Press, 1966), 177. Anselm writes:
No man besides him ever gave to God, by dying, what he was not necessarily going to lose at some time, or paid what he did not owe. But this man freely offered to the Father what he would never have lost by any necessity, and paid for sinners what he did not owe himself.” As we can see, the accent here falls heavily on the activity of the human nature.
 See Zwingli’s own discussion of the concept in Zwingli, 2:319-22; Also see August Baur, Zwinglis Theologie: Ihr Werden und Ihr System, 2 vols. (Zürich: Georg Olms Verlag, 1983-1984), 2:425, 460, 473, 484-510; Hägglund, 243-44; Gottfried Locher, Zwingli’s Thought: New Perspectives (Leiden: Brill, 1981), 173-6; Potter, 43, 305, 312-3, 336; Otto Ritschl, Dogmengeschichte des Protestantismus, 4 vols. (Göttingen: Vandenhoeck and Ruprecht, 1908-1927), 3:108-22; Reinhold Seeberg, Text-Book of the History of Doctrines, 2 vols. trans. Charles Hay (Grand Rapids: Baker Book House, 1977), 2:321, 324; Stephens, 112-17;
 Bente, 159; Ritschl, 2:325, 475, 482; Seeberg, 2:374; Vainio, 107-9.
 Evans, "Eutyches, Nestorius, and Chalcedon," in Evans, 243-7; Hägglund, 89-106; Kelly, 310-333; John Anthony McGuckin, St. Cyril of Alexandria: The Christological Controversy : Its History, Theology, and Texts (Crestwood, Ny: St. Vladimir's Seminary Press, 2000); Pelikan, 1:226-77; Cyril of Alexandria (New York: Routledge, 2000), 31-64; Seeberg, 1:261-5; Paul Tillich, A History of Christian Thought: From its Judaic and Hellenistic Origins to Existentialism, ed. Carl Braaten (New York: Simon and Schuster, 1968), 79-90.
 Leo the Great, Letter XXVIII, 4; NPNF, 12:41. Emphasis added. Leo goes in the same passage to strongly emphasize the duality of the two natures.
 See a good Lutheran critique of Leo in Robert Jenson, Systematic Theology, 2 vols. (New York: Oxford University Press, 1997-1999), 1:130-3; Steven Paulson, Lutheran Theology (Edinburgh: T & T Clark, 2011), 94-6.
 ST. 3a. q. 26, art. 1-2; BF, 50:206-13. See discussion in Davies, 321-38; Gratsch, 235-8.
 ST, 3a. q. 26, art. 2; BF, 50:212, 213. “Ad tertium dicendum quod, licet auctoritative peccatum auferre conveniat Christo secundum quod est Deus,tamen satisfacere pro peccato humani generis convenit ei secundum quod homo. Et secundum hoc dicitur Dei et hominum mediator.” Emphasis added.
 ST, 3a. q. 22, art. 3; BF, 50:144. “Ad primum ergo dicendum quod, licet Christus non fuerit sacerdos secundum quod Deus, sed secundum quod homo, unus tamen et idem fuit sacerdos et Deus.”
 ST, 3a. q. 18, art. 1; BF, 50:66.
Nam instrumentum inanimatum, sicut securis aut serra, movetur ab artifice per motum solum corporalem. Instrumentum vero animatum anima sensibili movetur per appetitum sensitivum, sicut equus a sessore. Instrumentum vero animatum anima rationali movetur per voluntatem eius, sicut per imperium domini movetur servus ad aliquid agendum, qui quidem servus est sicut instrumentum animatum, ut philosophus dicit, in I Politic. Sic ergo natura humana in Christo fuit instrumentum divinitatis ut moveretur per propriam voluntatem.
 ST, 3a. q. 18, art. 1; BF, 50:68.
 ST, 3a. q. 18, art. 4; BF, 50:76. “Ad tertium dicendum quod voluntas Christi, licet sit determinata ad bonum, non tamen est determinata ad hoc vel illud bonum. Et ideo pertinebat ad Christum eligere per liberum arbitrium confirmatum in bono, sicut ad beatos.”
 David Coffey, “The Theandric Nature of Christ,” Theological Studies 60. no 3 (1999), 208. Coffey describes the situation thus: “He [Aquinas] says that the humanity of Christ, the "moved" in this case, became the "instrument" of the divinity through his obedience freely rendered to the sovereign will of God.” Emphasis added.
 Ibid., 409. Coffey again puts it this way: “Hence what appears as a single, theandric operation of Christ is in reality two distinct operations working together in perfect communion.” Emphasis added. Also see discussion in Cross, 246-56; Jean-Pierre Torrell, Saint Thomas Aquinas, 2 vols., trans. Robert Royal (Washington, D. C.: The Catholic University of America Press, 2003), 2:128-30.
 ST, 3a, q. 46, art. 12; BF, 54:50-3.
 ST, 3a, q. 22, art. 3; BF, 50:146. “Et ideo, inquantum eius humanitas operabatur in virtute divinitatis, illud sacrificium erat efficacissimum ad delenda peccata.” Emphasis added.
 ST, 3a. q. 1, art. 2; BF, 48:14.
Ad secundum dicendum quod aliqua satisfactio potest dici sufficiens dupliciter. Uno modo, perfecte, quia est condigna per quandam adaequationem ad recompensationem commissae culpae. Et sic hominis puri satisfactio sufficiens esse non potuit, quia tota natura humana erat per peccatum corrupta; nec bonum alicuius personae, vel etiam plurium, poterat per aequiparantiam totius naturae detrimentum recompensare. Tum etiam quia peccatum contra Deum commissum quandam infinitatem habet ex infinitate divinae maiestatis, tanto enim offensa est gravior, quanto maior est ille in quem delinquitur. Unde oportuit, ad condignam satisfactionem, ut actio satisfacientis haberet efficaciam infinitam, ut puta Dei et hominis existens.
 ST, 3a, q. 1, art. 2; BF, 48:13. See a good summary of Aquinas’ atonement theology in Brian McDermott, Word Become Flesh: Dimension of Christology (Collegeville, MN: Liturgical Press, 1993), 220-3; Albert Patfoort. “Le vrai visage de la satisfaction du Christ selon St Thomas : une étude de la Somme Théologique” in Ordo sapientiae et amoris (Fribourg: Universitätsverlag Freiburg Schweiz, 1993), 247-65; Philip Quinn, “Aquinas on Atonement,” in Trinity, Incarnation, and Atonement (Notre Dame: University of Notre Dame Press, 1989), 153-77; Stump, 427-54.
 ST 3a. q. 18, art. 1; BF, 50:64-6. See Torrell, 2:128-30.
 ST, 3a. q. 25, art. 2; BF, 50:188-90:
Respondeo dicendum quod, sicut supra dictum est, honor adorationis debetur hypostasi subsistenti, tamen ratio honoris potest esse aliquid non subsistens, propter quod honoratur persona cui illud inest. Adoratio igitur humanitatis Christi dupliciter potest intelligi. Uno modo, ut sit eius sicut rei adoratae. Et sic adorare carnem Christi nihil est aliud quam adorare verbum Dei incarnatum, sicut adorare vestem regis nihil est aliud quam adorare regem vestitum. Et secundum hoc, adoratio humanitatis Christi est adoratio latriae.
 ST, 3a. q. 25, art. 2; BF, 50:191:
Alio modo potest intelligi adoratio humanitatis Christi quae fit ratione humanitatis Christi perfectae omni munere gratiarum. Et sic adoratio humanitatis Christi non est adoratio latriae, sed adoratio duliae. Ita scilicet quod una et eadem persona Christi adoretur adoratione latriae propter suam divinitatem et adoratione duliae propter perfectionem humanitatis.