Thomas Aquinas begins Summa Theologicae Part III with reflections on the Incarnation of Christ as the means by which God wills to bring about the proper end of human existence by overcoming sin. This section of the Summa reinvigorated Chalcedonian Christology in the West. Aquinas presents the Chalcedonian Decree [ST III.2.1.sc & III.2.2.sc], which was neglected by his contemporaries and predecessors like Peter Lombard, as the authoritative basis for orthodox Christology. Aquinas’s use of Chalcedon and the support of the Chalcedonian Christology of John of Damascus firmly established the unity of Christ’s divine and human natures without confusion or separation in the one eternal person of the Son. Aquinas’s emphasis on the hypostatic union did not diminish the distinctiveness of Christ’s two natures.
For instance, Aquinas repeatedly affirms that Christ remains omnipresent according to his divine nature even while taking to himself a finite and circumscribed human nature, what will come to be known as the extra Calvinisticum in later centuries. Christ’s presence extra carnem, beyond the flesh, in no way separates the person of Christ but rather secures his presence with the Church. Aquinas repeatedly affirms that Christ remains omnipresent according to his divine nature even while taking to himself a finite and circumscribed human nature. Click To Tweet
Aquinas explains the extra carnem by reflecting on two descents, as noted by McGinnis, the “coming down” of the incarnation and his descent into hell on Holy Saturday. To this, we must add Christ’s ascent into heaven. The idea that unites these events is the relation of the omnipresent Divine Son to his spatially defined human nature at points of cosmological transition. The Son according to his divine nature remains omnipresent and transcendent of all space as he is one with the Father and the Holy Spirit, while the human nature of Christ and its parts, can come to be and move locally in created space. These moments of descent and ascent in the incarnate life of the Logos are explicated by Aquinas according to the mystery of the Incarnation as defined by Chalcedon. The eternal Son remains both true God, vere deus, and becomes true man, vere homo, by the assumption of the human nature into his preexistent personhood.
A key concept for understanding Aquinas’ Christology, and the extra carnem in particular, is that of a “mixed relation.” Because of God’s aseity and immutability, he relates to creation without taking on any new properties, which reside only on the creaturely side of the relation. While all relations between the triune God and creation have this “mixed” or asymmetric quality, the Son’s assuming human nature into his own person is unique. Christ became everything that we are without ceasing to be everything he was in eternal communion with the Father and Spirit. As Thomas explains,
“To be man is newly predicated of God without any change in Him, by a change in the human nature, which is assumed to a Divine Person. And hence, when it is said, ‘God was made man,’ we understand no change on the part of God, but only on the part of the human nature.”
This is true of all aspects of the Christ incarnation but particularly clarifies the relationship of the omnipresent divine nature and the particular spatially limited nature of the human body. When scripture says, God was made man, we understand no change on the part of God, but only on the part of the human nature. -Thomas Aquinas Click To Tweet
The Incarnational Decent and Divine Omnipresence
Jesus repeatedly describes his relationship with the Father as having been “sent” (Mark 9:38; Luke 9:48; 10:16, John 5:23-24; 16:5 etc.) or “coming down” from heaven (John 3:13; 6:50-51, etc.). Do such statements mean that Christ moved according to his divine nature such that he was in a new place or ceased to be where he once was? Christ’s unity with the Father and the Spirit and the attributes of the divine nature utterly prohibits this. Aquinas explains such a sending according to the logic of mixed relation, “for since the divine persons are everywhere by essence, presence, and power, a [divine] person is said to be sent according to this, that he begins to be in a creature in a new mode through some new effect.”
Therefore, the Son does not first become present in creation with the Incarnation, since as the eternal Logos he is always present. For Aquinas the “incarnational decent” must be understood as the assumption of the human nature by the divine person of the Son and not some coming-to-be or a change of location. Therefore, Christ in his new incarnate state is both localized in his human nature and body and everywhere as the eternal Word of the Father extra carnem. Thomas commenting on John 1:14 states,
He descended from heaven in such a way as yet to be in heaven. For he came down from heaven without ceasing to be above, yet assuming a nature which is from below. And because he is not enclosed or held fast by his body which exists on earth, he was, according to his divinity, in heaven and everywhere.
While the human nature of Jesus Christ is in the closest possible relation with God, i.e. being hypostatically united to the Second Triune Person, yet he is not diminished or contained. The infinite remains beyond the bounds of the finite, even in that place were the infinite God reveals himself most fully in finite existence. “Not even in the union by personal being does the human nature comprehend the Word of God or the Divine Nature, for although it was wholly united to the human nature in the one Person of the Son, yet the whole power of the Godhead was not circumscribed by the human nature.”  Thomas ties this point into the Western tradition through appealing to Augustine.
Christ’s divine nature does not shrink to the limits of bodily existence nor does his body fundamentally change. The body that Christ took to himself was a real human body like our own, local and circumscribed. Aquinas diligently demarcates the Creator/creature distinction in his articulation of Christ’s person and natures such that they remain unconfused and unchanged as Chalcedon confessed:
“So likewise the flesh is said to be deified…not by change, but by union with the Word, its natural properties still remaining, and hence it may be considered as deified, inasmuch as it becomes the flesh of the Word of God, but not that it becomes God.”
The human nature assumed by the Word of God is ennobled, but the Word of God is not changed. -Thomas Aquinas Click To Tweet Here Aquinas seeks to secure the integrity of both the human and divine natures in Christ, while acknowledging the closeness of their union. While Aquinas uses the language of deification it is qualified by a firm understanding of the distinct perfections of both God and man. The perfect human essence is not changed into divinity but united in differentiation with the Divine. “The human nature assumed by the Word of God is ennobled, but the Word of God is not changed.”
Where was Jesus during the three days? Totus and Totum
Aquinas uses his understanding of the Chalcedonian formula to address a question regarding Christ’s presence between his death on the Cross and the Resurrection, the three days or triduum. Christ promised the thief on the Cross that very day he would be with him in paradise (Luke 23:43). And yet on the same evening, Christ was in the tomb and, according to tradition, Christ has descended into hell. What are we to say regarding Christ’s presence during this period? Is he in the tomb, in hell, or in paradise?
One must answer in some sense all three, yet this cannot be said simply but must be specified. Peter Lombard presents this question in his Sentences and addresses it using a distinction between Christ being present “totus” vs. “totum.” These Latin adjectives both mean “whole” but totus is in the masculine and totumin the neuter grammatical gender; according to both Lombard and Aquinas when discussing Christ, the masculine adjective refers to the person while the neuter refers to the natures.
A similar mode of expression in English is used when persons are referred to with “he/she” while inanimate objects generally are called “it.” Thus, the question can be reframed as the whole Christ (totus Christus) vs. the whole of Christ (totum Chrsti). The whole Christ (totus) is the person of the eternal Son who is everywhere by dint of the divine omnipresence and is the one who is joined with the human nature everywhere (even if that nature considered on its own is not in all places). The whole of Christ (totum Christi), i.e. Christ considered on the level of natures and the parts of the human nature, is not in all places since this specifically references the human body, which is spatially finite.
So, where was Jesus during the three days? We can give a threefold answer. According to his person and divine nature, the whole Christ is everywhere, and in that sense, he is with the thief in paradise. Likewise, the whole Christ was in the tomb since the body was united with him and according to divine omnipresence. The whole Christ was also in hell by his human soul and divine nature.
However, the whole of Christ, considered on the level of parts, was not in the tomb or in hell since only part of the human nature was present there. Understanding the presence of Christ from death to resurrection not only helps us to work through a particular exegetical difficulty, but also reminds us that the unity of the person is not overcome by the distinctiveness of Christ’s natures. The hypostasis/person of the Son is never absent from his Creation but is always present as the divine Lord even through death. Though his human nature itself is not omnipresent, the One who has assumed human nature to himself as Emmanuel is present with his people even on Holy Saturday and still even as he ascended into heaven.
The Ascension and the Whole Christ
Aquinas also reflected on the presence of Jesus’ extra carnem regarding the Ascension by which he, according to his physical body and human nature, “went away.” Aquinas asks how to understand such language regarding Christ who is both God and Man. In what sense does Christ ascend as the apostles look on and in what sense does he remain with us until the end of the age?
In the Summa, Aquinas asks whether it is appropriate to claim that Christ ascends according to his divine nature. This depends on what we understand by “according to,” which can mean with respect to “condition” or “cause.” Aquinas argues that Christ cannot ascend according to the condition of being divine since his divine nature is always everywhere, as discussed above. However, it is appropriate to discuss Christ’s divine nature as the cause of his ascension, along with the Father and the Holy Spirit, because it was not by a human power that he ascended into heaven.
The Ascension, according to Aquinas, is better predicated according to Christ’s human nature: “Yet the Ascension is in keeping with Christ according to His human nature, which is limited by place, and can be the subject of motion. In this sense, then, we can say that Christ ascended into heaven as man, but not as God.” Christ does not need to go to the Father in his divinity since the Incarnation does not absent him from his eternal union within the Triune essence. However, the human nature now glorified must be taken into heaven to prepare a way for the elect. Yet, the “who” ascended is not the human nature but the person of the eternal Son who has assumed flesh. “He who ascended is the same as He who descended … The God-man.” The acting agent in the ascension is nothing less than Jesus Christ.
Therefore, using the terminology we employed above, the whole Christ ascended: the person as the active subject, the human nature by motion, and the divine nature as cause; the whole of Christ did not transition from heaven to earth, strictly speaking, since this only applies to the human nature. While Christ is physically absent via the ascension he remains with his people via “the spiritual presence of Christ, his presence in his divinity and in the sacraments.” As Aquinas paraphrases Jesus’s promise to be with his people after the ascension, “although I will leave you in body, I am still spiritually with you.”
Thomas Aquinas, a beacon and a bridge
Thomas Aquinas calls us to the fundamental mystery of the Incarnation, that the eternal Son became man without in any way ceasing to be God. While this is seen across Aquinas’s treatment of Christology, it is brought into sharp relief through his reflection on Christ beyond the flesh. Aquinas’s Christology is both a beacon and a bridge. A beacon in that it guides thinkers successfully through the channel of Chalcedonian orthodoxy and away from the errors of confusing the natures (Eutychianism) or duplicating the person (Nestorianism). At the same time, he offers a bridge bringing together Augustine, Chalcedon, and John of Damascus in a potent synthesis that is carried forward into the early modern period.Calvin makes use of Aquinas's totus/totum distinction to explain the nature of Christ’s presence after the ascension. Click To Tweet
In addition to the obvious influence on the Roman Catholic Church, Aquinas was used in Reformed Christology as well. Calvin makes use of the totus/totum distinction to explain the nature of Christ’s presence after the ascension, although he generically references “the scholastics.” However, Peter Martyr Vermigli explicitly invokes Thomas in his Dialogue on the Two Natures in Christ to support the unity of the person of Christ and the circumscription of the body against Lutheran claims of an omnipresent human nature.
Thomas, along with the Christian tradition, reminds us that although we live in the time of Christ physical absence by faith and not sight, Christ will never forsake his Church. “Although Christ’s bodily presence was withdrawn from the faithful by the Ascension, still the presence of His Godhead is ever with the faithful.”
 Corey L. Barnes, “Thomas Aquinas’s Chalcedonian Christology and Its Influence on Later Scholastics,” The Thomist: A Speculative Quarterly Review 78, no. 2 (2014): 189–217; Jean-Pierre Torell, OP, “Saint Thomas and His Sources,” in The Oxford Handbook of the Reception of Aquinas, ed. Matthew Levering and Marcus Plested (Oxford; New York: Oxford University Press, 2021), 11–12.
 Andrew M. McGinnis, The Son of God Beyond the Flesh: A Historical and Theological Study of the Extra Calvinisticum, T & T Clark Studies in Systematic Theology 29 (London: Bloomsbury T&T Clark, 2014), 47.
 It should be noted that Thomas sees the circumscribed nature of the human body as part of its created nature and not as a defect created by the fall or that will be overcome in humanity’s glorification. For instance, the finitude of the body is not contained in Thomas’s discussion of the defect that Christ assumed from the body and overcame. ST III. Q.14. See also Thomas’s argument that God cannot make a body be in more than one place in Quodlibetal III.1.2.
 For the discussion of a “mixed relation” in connection with the Incarnation in Thomas see Michael Gorman, Aquinas on the Metaphysics of the Hypostatic Union (Cambridge, United Kingdom ; New York, NY: Cambridge University Press, 2017), 62–72; Dominic Legge, The Trinitarian Christology of St Thomas Aquinas, Reprint edition (Oxford, United Kingdom ; New York, NY: Oxford University Press, 2018), 19–21.
 ST III.6.6.ad.2.
 Contra errorres Graecorum I, c. 14. Quoted in Legge, The Trinitarian Christology of St Thomas Aquinas, 18.
 Thomas Aquinas, Commentary on the Gospel of John, Books 1-5, trans. Fabian R. Larcher and James A. Weisheipl, vol. 1 (Washington, D.C: The Catholic University of America Press, 2010), 177–78.
 ST III.10.1.ad.2.
 “Hence Augustine says (Ep. ad Volusian. cxxxvii): ‘I would have you know that it is not the Christian doctrine that God was united to flesh in such a manner as to quit or lose the care of the world’s government, neither did He narrow or reduce it when He transferred it to that little body.”” Quoted in ST III.10.1.ad.2.
 ST III.2.1.ad.3.
 ST III.2.6.ad.1.
 Lombard, The Sentences Book 3, 94–96, [Distinction XXII.2-3].
 For a discussion of this distinction throughout the Middle Ages see E. David Willis, Calvin’s Catholic Christology: The Function of the so-Called Extra Calvinisticum in Calvin’s Theology, Studies in Medieval and Reformation Thought 2 (Leiden: E J Brill, 1966), 34–44.
 ST III.52.3.ad.3
 “Consequently, it must be affirmed that during the three days of Christ’s death the whole Christ was in the tomb, because the whole Person was there through the body united with Him, and likewise He was entirely in hell, because the whole Person of Christ was there by reason of the soul united with Him, and the whole Christ was then everywhere by reason of the Divine Nature.” ST III. 52.3. co
 ST III.57.2.co.
 ST III.57.2.co.
 ST III.57.2.ad.2.
 Thomas Aquinas, Commentary on the Gospel of John, Books 13-21, trans. Fabian R. Larcher and James A. Weisheipl, vol. 3 (Washington, D.C: The Catholic University of America Press, 2010), 41.
 For the general reception of Thomas among the Reformed in the 16th century see David Sytsma, “Sixteenth-Century Reformed Reception of Aquinas,” in The Oxford Handbook of the Reception of Aquinas, ed. Matthew Levering and Marcus Plested (Oxford; New York: Oxford University Press, 2021), 121–43.
 John Calvin, Institutes of the Christian Religion, ed. John T. McNeill, trans. Ford Lewis Battles (Louisville, KY: Westminster John Knox Press, 1960), IV.17.30. For this topic more broadly in Calvin see Willis, Calvin’s Catholic Christology, 31–33.
 Peter Martyr Vermigli, Dialogue on the Two Natures in Christ, ed. John Patrick Donnelly S.J., The Peter Martyr Library 2 (Kirksville, MO: Sixteenth Century Journal Publishers, 1995), 100–101. For more on Vermigli and this doctrine see K.J. Drake, The Flesh of the Word: The Extra Calvinisticum from Zwingli to Early Orthodoxy, Oxford Studies in Historical Theology (Oxford, New York: Oxford University Press, 2021), 140–204.
 ST III.57.2 ad.3.