As we have already noted, both theologians accept the Christological boundaries established in the first six ecumenical councils. Therefore at the beginning of De Duabus Naturis, Chemnitz outlines a series of general definitions and explanations of the hypostatic union derived from these early councils and the Church Fathers. Following the formulation of the fourth ecumenical council, Jesus is both true God and true man in one person (or hypostasis). Within this union of divine and human natures, neither nature is mixed with or transmuted into the other. Although both natures retain their essential integrity, when considered in the concrete unity of the hypostatic union, neither can be spoke of in isolation from the other.
Following the third ecumenical council implicitly and fifth ecumenical council explicitly, Chemnitz asserts that we must view the Second Person of the Trinity as the subject of the Incarnation. The divine person of the Logos incorporates a non-personal or “anhypostatic” human nature into his own pre-existent hypostasis. Chemnitz explains that in the human nature of Christ we are confronted with “one particular individual unit (massa) of human nature.” This does not mean that Christ’s humanity lacks anything that is essential to human nature; rather what it lacks is any independent reality as an individual person or center of identity. Christ’s human nature subsists in the pre-existent person of the Son, within which it is incorporated (enhypostasis). Moreover, since the human nature possesses its center of identity and reality through its subsistence in the person of the Logos, what befalls it must be viewed (when considered in the concrete) as befalling the person of the Logos. This is true because the unity of the person of Christ is not merely notional or rhetorical, but hypostatic and real. For this reason (following the third ecumenical council) Mary must be referred to as the Mother of God or the “God-bearer” (Θεοτόκος). In giving birth to Christ, she gave birth to his total theandric person.
Lastly, contrary to the claims of some, Chemnitz fully accepts the teaching of the sixth ecumenical council that Christ possesses two wills (δυοθελητισμός). This does not mean that Christ has two centers of identity or consciousness that are potentially in conflict with one another. Rather, what it means is that Christ lacks nothing that is essential to human nature, including a human volition. As Maximus the Confessor argued during the Monothelite controversy, if Christ lacked anything that is essential to humanity, it would be impossible for him to redeem other humans. The claim of the sixth ecumenical council is therefore merely reaffirmation of Chalcedon’s insistence on Christ’s full divinity and humanity.
In Tertia Pars of the Summa, Aquinas gives roughly the same list of propositions and distinctions discussed above. The major difference is that in the treatment of these aspects of the hypostatic union is more drawn out and detailed than in De Duabus Naturis. The Angelic Doctor begins by affirming that Jesus is both true God and man, subsisting in a single concrete person. In the Incarnation, both natures retain their integrity and the essential qualities of each nature are left undisturbed. The hypostatic union may be spoken of in analogy with the union of grace, though unity between the two natures far exceeds any other union between God and his creatures.Despite this strong affirmation of the unity of the person of Christ, we will later see that choice of the analogy of the union of grace is rather telling for how Aquinas will later describe the interaction between the two natures.
Lastly, following the mainstream of the Latin tradition, Aquinas asserts that the human nature in Christ is united with the divine person of the Logos through the medium of his soul. This latter assertion, though not discussed in Chemnitz, is rejected by many Lutheran theologians on the grounds of its rationalistic nature and the lack of biblical evidence.
 Chemnitz, 1-4; Preus, 29-36.
 Chemnitz, 26, 38-9, 47, 62-4; Preus, 83, 113, 131, 172-3. For the text of Chalcedon see Tanner, 1:83. For discussions of Chalcedon see J. N. D. Kelly,Early Christian Doctrines (New York: Harper and Brothers, 1958), 339-342, 406; ; John Meyendorff, Christ in Eastern Christian Thought (Washington: Corpus Book, 1969), 12-6; Jaroslav Pelikan, The Christian Tradition: A History of the Development of Doctrine, 5 vols. (Chicago: University of Chicago Press, 1971- 1989), 1: 263-6; Linwood Urban, A Short History of Christian Thought (New York: Oxford University Press, 1995), 74-5, 88-9, 91, 93-94,98-100.
 Chemnitz, 18-20; Preus, 68-72. For the text of the fifth ecumenical council see Tanner, 1:107-22. See fifth ecumenical council in Aloys Grillmeier,Christ in Christian Tradition, Pt. 2, vol. 2 (Louisville, Ky: Westminster-John Knox, 1995), 341, 387, 402- 10, 419-62, 463. Meyendorff, 38-40, 59-64; Pelikan, 2:29-30.
 Chemnitz, 2; Preus, 30. “Sed naturam humanum assumpsit consideratam in una certa individua massa . . .”
 See I. A. Dorner, Entwicklungsgeschichte der lehre von der person Christi von den ältesten zeiten bis auf die neueste dargestellt, 3 vols. (Stuttgart: Verlag von Samuel Gottlieb Liesching, 1845-1856), 1:106; Gottfried Thomasius, Die christliche dogmengeschichte als entwicklungs-geschichte des kirchlichen lehrbegriffs, 2 vols. (Erlangen: A. Diechert, 1874-1876), 1:371. Also see discussion of Chemnitz’s acceptance of the sixth ecumenical council and criticism of these two figures’ characterizations in: Tom Hardt, “The Sixth Ecumenical Council,” in Mysteria Dei: Essays in Honor of Kurt Marquart, eds. Paul McCain and John Stephenson (Ft. Wayne, IN: Concordia Theological Seminary Press, 2000), 67-78.
 Chemnitz, 87-90; Preus, 233-9. For the text of the sixth ecumenical council see Tanner, 1:124-30. For sixth ecumenical council see Pelikan, 2:70-5; George Ostrogorski, History of the Byzantine State (New Brunwick, NJ: Blackwell, 1969), 127.
 See Demetrios Bathrellos, The Byzantine Christ: Person, Nature, and Will in the Christology of Saint Maximus the Confessor (Oxford, UK: Oxford University Press, 2004); Meyerdorff, 42-3, 99-115; Pelikan, 2:8-36; Janet Williams, “Pseudo-Dionysius and Maximus the Confessor,” in The First Christian Theologians: An Introduction to Theology in the Early Church, ed. G. R. Evans, (Malden, Mass: Blackwell Publishing, 2004), 193-9. Andrew Louth, Maximus the Confessor (New York: Routledge, 1996); Lars Thunberg, Microcosm and Mediator: The Theological Anthropology of Maximus the Confessor (Chicago: Open Court, 1995).
 See brief description of Aquinas’ acceptance of the parameters of the ancient councils in Richard Cross, The Metaphysics of the Incarnation: Thomas Aquinas to Duns Scotus (Oxford, UK: Oxford University Press, 2005), 51-64; Michael Gorman, “Christ as Composite according to Aquinas” Traditio 55 (2000): 143-57; idem, “Uses of the Person-Nature distinction in Thomas's Christology,” Recherches de Théologie et Philosophie Médiévales 67, no 1 (2000): 58-79; Herman Otto Pesch, Thomas von Aquin: Grenze und Größe mittelalter Theologie (Mainz: Matthias-Grünewald Verlag, 1989), 334-5; Eleanore Stump, Aquinas (New York: Routledge, 2005), 407-26; Joseph Wawrykow, The Westminster Handbook to Thomas Aquinas (Louisville, Ky: Westminster John Knox, 2005), 71-3.
 ST, 3a. q.2, art.1-2; BF, 48:35-47. Also see Edward Gratsch, Aquinas’ Summa: An Introduction and Interpretation (New York: Alba House, 1985), 217-8; Joseph Wawrykow, “The Hypostatic Union” in The Theology of Thomas Aquinas, ed. Joseph Wawrykow and Nik Van Nieuwenhove (Notre Dame: Notre Dame University Press, 2005), 222-51. Brian Davies, The Thought of Thomas Aquinas (Oxford, UK: Oxford University Press, 1992), 297-307.
 ST, 3a. q.2, art.1-2; BF, 48:35-47.
 ST, 3a. q.2, art.9; BF 48:73-9.
 Peter Lombard, The Sentences, 4 vols., trans. Giulio Silano (Toronto: Pontifical Institute of Medieval Studies, 2007-2010), 3:8.
 ST, 3a. q.6, art.4; BF, 48:165. “Ad primum ergo dicendum quod caro humana sortitur esse per animam.”
 For example, this is the judgment of Francis Pieper. See Francis Pieper, Christian Dogmatics, 3 vols. (St. Louis: Concordia Publishing House, 1953-1957), 2:99-100.