Pope St. Leo the Great’s Christology
The Council of Chalcedon was summoned by Emperor Marcian during the autumn of 451 to respond to an extreme model of monophysite Christology that threatened to eclipse the moderate teaching of Ephesus. The Fathers of Chalcedon sought to establish a normative teaching on the Incarnation, confessing “one and the same Son, our Lord Jesus Christ, the same perfect in human nature, truly God and the same with a rational soul and a body truly man . . .acknowledged in two natures . . . combining in one Person and substance (hypostasis), not divided or separated into two persons, but one and the same Son only begotten God Word, Lord Jesus Christ.” While the Council was profoundly influenced by the Christologies of Cyril of Alexandria and John of Antioch, it was under the sway of Pope Leo the Great’s Tome to Flavian that the majority of conflicting parties at Chalcedon were reconciled. The bishop of Rome provided the decisive polemic against the false Christological interpretations threating the orthodox faith, marking the first instance of a Western solution to a peculiarly Eastern controversy. But Leonine Christology was not exhausted through its service to the Chalcedonian formula. Rather, as a system it goes much further than the Council with regard to the prerogatives of Christ’s natures, exemplifying a refined Latin tradition of the twofold consubstantiality of Christ.
The starting point for Leonine Christology is the doctrine of the Son’s consubstantiality with the Father as promulgated at the First Council of Nicaea. The Son of God was “begotten as coeternal from the eternal—not later in time or inferior in power or dissimilar in splendor or different in essence.” Leo emphasizes that Christ was born of both God and Mary, and thereby possessed two distinct natures (duae naturae), divine and human, within the unity of his person (persona). Each respective nature possesses its own properties and activities accordinly, yet “both natures and substances are kept intact yet and come together in one person.” However, Leo’s prime concern does not lie solely with his explication of an ontological Christology. His thought is always profoundly orientated toward the salvific economy of Christ: God became man so that humanity could be saved. For the bishop of Rome, redemption is rooted in the very person of Christ; precisely as one persona and two naturae, he is the efficacious savior of humanity: “In this way, as our salvation requires, one and the same mediator between God and men, the man Jesus Christ, can at one and the same time die in virtue of the one nature and, in virtue of the other, be incapable of death.” Because of his dual consubstantiality, Christ is capable of mediating divine salvation to sinful humanity.
Leo’s soteriological concerns emerge even more explicitly in his Letter to Emperor Leo I (Ep. 165), dispatched in August 453 during the struggle for the implementation of the Chalcedonian norm. This letter, commonly dubbed the “Second Tome,” contains Leo’s most developed Christological exposition. The mediatorship of Christ is again brought to the fore, as Leo asserts that death can be conquered only by a divine and human agent. The “Second Tome” locates the moment of salvation in the voluntary death of Christ, who is both eternal priest and perpetual sacrifice. As a guiltless victim, free from the stain of original sin, Christ was not subject to the debt of sin and was capable of overcoming the dominance of Satan. Because the Son of God assumed a concrete human nature, all humanity is incorporated into the destiny of Christ, most particularly his resurrection—all have died and risen by virtue of their bond to Christ’s humanity. Thus, if Christ possesses only one nature (monophysitism), or if the natures operate as two loosely joined subjects (Nestorianism), his mediatorship is depreciated and salvation becomes an impossibility.
 For background on the events immediate preceeding the Council of Chalcedon, see John A. McGuckin, St. Cyril of Alexandria and the Christological Controversy (Crestwood, NY: St. Vladimir’s Seminary Press, 2004); Basil Studer, Trinity and Incarnation: The Faith of the Early Church (Collegeville, MN: Liturgical Press, 1993), 211-12; Donald Goergen, The Jesus of Christian History, vol. 3 of A Theology of Jesus (Collegeville: Litrugical Press, 1992), 136-41.
 I quote from the English translation in Henry Denzinger, ed., The Sources of Catholic Dogma (Fitzwilliam, NH: Loretto, 1955), no. 148.
 See Jaroslav Pelikan, The Emergence of the Catholic Tradition (100-600), vol. 1 of The Christian Tradition: A History of the Development of Doctrine (Chicago: University of Chicago Press, 1971), 263-4.
 Tomus ad Flavianum, 2. I quote from the English translation in Richard A. Norris, ed. and trans., The Christological Controversy (Philadelphia: Fortress Press, 1980), 146.
 Tomus ad Flavianum, 3; Norris, 147.
 See Aloys Grillmeier, From the Apostolic Age to Chalcedon, vol. 1 of Christ in Christian Tradition (New York: Sheed and Ward, 1965), 466.
 Tomus ad Falvianum, 3; Norris 148.
 Ep. 165 ad Leon. aug.
 See the discussion on the epistle in Grillmeier, Reception and Contradiction: The Development of the Discussion about Chalcedon from 451 to the Beginning of the Reign of Justinian, vol. 2.1 of Christ in Christian Tradition (London: Mowbray, 1987), 149-72.