The Incarnation and the Two Natures

For Leo the Great the doctrine of the two natures is never an abstract Christological puzzle but the very ground of salvation: only because Christ is at once true God and true man can He save humanity at all. This file expounds Leo’s soteriology of the Incarnation — the admirabile commercium or “wondrous exchange” by which God became what we are that we might become what He is; the dignity of the human creature “made a partaker of the divine nature” (Sermon 21.3); the redemptive arc traced through his liturgical preaching from Nativity to Pentecost; the communicatio idiomatum and the unity of the one suffering Person; and the convergence of Leo’s Latin idiom with the Eastern doctrine of theosis.

Tradition: Pan-Orthodox (Church Father / Latin)

Salvation Grounded in the Two Natures

Leo’s Christology and his soteriology are a single doctrine viewed from two sides. He never argues the two natures as a speculative refinement; he argues them because the integrity of each is the condition of redemption. The logic is as plain as it is rigorous: only true God has the power to save, and only a true man can be the representative who, on humanity’s behalf, offers the obedience and bears the death by which humanity is restored. Were Christ less than fully divine, His saving act would lack omnipotent efficacy; were He less than fully human — were His flesh, as Eutyches taught, not consubstantial with ours — then it would not be our nature that was healed, and the Cross would redeem no one (cf. Ep. 28.3–4; trans. Feltoe, NPNF 2nd ser. 12; Wessel, Leo the Great and the Spiritual Rebuilding of a Universal Rome, 220–238). This is why Leo can write, in the Tome, that “to pay off the debt of our condition, the inviolable nature was united to a nature that could suffer; so that… one and the same Mediator between God and men, the Man Christ Jesus, could both die in respect of the one and not die in respect of the other” (Ep. 28.3; trans. Feltoe, NPNF 2nd ser. 12). The whole soteriology turns on that union: a Mediator who is both the God offended and the man indebted.

Leo presses the point against both of the great errors. Against Nestorius, who divided the natures into something like two subjects, Leo insists that the one who saves must be a single Person, lest the manhood’s obedience be the work of a mere man set alongside God rather than of God-made-man. Against Eutyches, who confused or absorbed the manhood, Leo insists that the humanity remain whole and undiminished, “complete in what was His own, complete in what was ours” (Ep. 28.4; trans. Feltoe, NPNF 2nd ser. 12), for a humanity swallowed up in divinity could neither suffer nor stand as our representative. The salvific middle path between dividing and confusing is therefore the same as the dogmatic one (see The Tome to Flavian).

The Wondrous Exchange

The heart of Leo’s redemptive vision is the admirabile commercium — the “wondrous” or “marvelous exchange” by which, in the Incarnation, the Son of God takes our lowliness in order to give us His exaltation. The formula belongs to a long patristic lineage — Irenaeus, Athanasius, and Augustine all teach the exchange before Leo — and the phrase itself was fixed in the Latin liturgy as the antiphon O admirabile commercium sung in the Christmas Octave [Disputed: the antiphon is medieval and of uncertain authorship; it is not a composition of Leo, though it crystallizes the doctrine his Nativity sermons preach. It is sometimes loosely attributed to him in popular sources, but no critical edition assigns it to Leo (cf. the liturgical antiphon’s transmission; Neil, Leo the Great, 47–60)]. What is Leo’s own is the incomparable rhetorical force he gives the exchange from the pulpit. In the Christmas sermons he returns again and again to the paradox that “the Creator of men deigned to become man” so that we might “be found in His nature whom we worship in ours” (Sermon 27.1–2; CCSL 138; trans. Freeland & Conway, FOTC 93). The descent of God is for the sake of the ascent of man: God assumes the human that the human may share in the divine.

The classic compressed statement of the exchange comes at the opening of Sermon 21, where Leo proclaims that “the Son of God… took on Him the nature of the human race, in order to reconcile it to its Author” (Sermon 21.2; CCSL 138; trans. Freeland & Conway, FOTC 93). This is the Western voice of the same logic Athanasius had given the East — “He became man that we might become god” (On the Incarnation 54) — and Leo’s idiom is unmistakably continuous with it.

“Recognize Your Dignity”

The most celebrated single sentence of Leo’s preaching draws the exchange into a direct moral summons. In the third section of his first Nativity sermon he exhorts: “Christian, acknowledge your dignity, and becoming a partner in the Divine nature, refuse to return to the old baseness by degenerate conduct” — Agnosce, o Christiane, dignitatem tuam, et divinae consors factus naturae, noli in veterem vilitatem degeneri conversatione redire (Sermon 21.3; CCSL 138 / PL 54:192–193; trans. Feltoe, NPNF 2nd ser. 12). The phrase consors divinae naturae is a direct citation of 2 Peter 1:4 — “partakers of the divine nature” — and it shows Leo grounding the believer’s exaltation squarely in the Incarnation: because God has taken our nature, the baptized may be called a sharer in His. The dignity is not native but conferred; it is the gift of the wondrous exchange made personal in each Christian, and its corollary is ethical — having been raised, one must not “return to the old baseness.” Here Leo’s soteriology of elevation and his pastoral ethics are one motion: redemption is both healing of the fallen creature and elevation of it into a participation that exceeds its first estate.

The Redemptive Arc in the Sermons

Leo’s soteriology is not laid out in a treatise but distributed across the liturgical year, so that the saving work of Christ is preached as it is celebrated. The arc runs through the festal cycle. The Nativity sermons (Sermons 21–30; CCSL 138; trans. Freeland & Conway, FOTC 93) proclaim the Incarnation itself as the inauguration of salvation — God made visible, the exchange begun. The Passion sermons, a remarkable and substantial series preached in Holy Week (the De Passione sermons, Sermons 52–70; CCSL 138A; e.g. Sermon 54, Palm Sunday 442; Sermon 59; trans. Feltoe, NPNF 2nd ser. 12), expound the Cross as the place where the two natures together accomplish redemption: the Word performs what is the Word’s, the flesh undergoes what is the flesh’s, and the one Person pays the debt. The Resurrection and Ascension sermons (Sermons 71–73; CCSL 138A; trans. Feltoe, NPNF 2nd ser. 12) carry the assumed humanity into glory — Leo teaches that in the Ascension our nature is enthroned at the right hand of the Father, the exchange completed in the lifting-up of man. The cycle closes at Pentecost (Sermons 75–77; CCSL 138A; trans. Feltoe, NPNF 2nd ser. 12), where the Spirit applies to the Church the redemption the Son has won. Read in sequence, the sermons present a single continuous economy: Incarnation, Passion, Resurrection, Ascension, the Spirit — the whole arc by which “what was visible in our Redeemer has passed over into the sacraments” (Sermon 74.2; CCSL 138A; trans. Feltoe, NPNF 2nd ser. 12; cf. Wessel, 169–197 on Leo’s homiletic theology).

The One Person Who Suffers

Because there is one Person in the two natures, the properties of each may be predicated of the single subject — the communicatio idiomatum, the “communication of idioms.” This is, for Leo, not a verbal nicety but the guarantee of redemption: it is truly God’s own Son who is born of the Virgin, who hungers and weeps, who is crucified and dies, and who is raised — “the same one” throughout (Ep. 28.4–5; trans. Feltoe, NPNF 2nd ser. 12; cf. Grillmeier, Christ in Christian Tradition, vol. 1, 534–539). When Leo says passus est — “He suffered” — the subject of the verb is the one Person of the Word, suffering in and according to the flesh while remaining impassible in the Godhead. The death that redeems is therefore neither the death of a mere man (which could not save) nor a suffering of the bare divinity (which is impossible), but the death of the incarnate Son, undergone in the nature that can die and offered by the Person whose dignity gives it infinite worth. The unity of the suffering Person is thus the linchpin of the whole soteriology: only because the one who suffers is the Son of God does His human death have saving power for all.

Convergence with Eastern Theosis

Leo’s teaching that the Christian is “made a partaker of the divine nature” places his Latin soteriology in close convergence with the Eastern doctrine of theosis, deification. The shared root is the exchange-formula common to Athanasius and Leo alike: God became man that man might be made god by grace. Leo’s idiom is his own — juridical and participatory rather than energetic, lacking the later essence/energies articulation of the Greek tradition — and he speaks of consortium and dignitas where the East speaks of theosis and union with the divine energies. But the underlying movement is the same: salvation is not merely pardon but real participation in the divine life, the elevation of the redeemed creature into communion with God (cf. Meyendorff, Christ in Eastern Christian Thought, 23–46; Neil, Leo the Great, 47–60). Leo is thus a Western witness to a pan-Orthodox conviction, expressing in the vocabulary of Rome the same mystery the Greek Fathers named deification (see Theosis). The “wondrous exchange” and “becoming partakers of the divine nature” are the Latin face of one shared patristic faith.

References

Leo the Great, Sermons (Tractatus), CCSL 138 and 138A, ed. Antoine Chavasse (Turnhout: Brepols, 1973); PL 54.
Leo the Great, Sermon 21 (On the Nativity), Sermon 27 (On the Nativity), Sermon 54 (On the Passion), Sermon 74 (On the Ascension); trans. Charles Lett Feltoe, Letters and Sermons of Leo the Great, NPNF 2nd ser. vol. 12 (1895); and trans. Jane Patricia Freeland & Agnes Josephine Conway, St. Leo the Great: Sermons, Fathers of the Church 93 (Washington, DC: CUA Press, 1996).
Leo the Great, Tome to Flavian (Ep. 28), PL 54:755–782; trans. Feltoe, NPNF 2nd ser. vol. 12.
Athanasius, On the Incarnation 54 (PG 25:192B).
2 Peter 1:4 (Scripture).
Susan Wessel, Leo the Great and the Spiritual Rebuilding of a Universal Rome (Leiden: Brill, 2008).
Bronwen Neil, Leo the Great, The Early Church Fathers (London: Routledge, 2009).
Aloys Grillmeier, Christ in Christian Tradition, vol. 1, 2nd rev. ed. (London: Mowbray, 1975).
John Meyendorff, Christ in Eastern Christian Thought (Crestwood, NY: SVS Press, 1975).