But both sets of properties and activities are attributed to the One Person of the Word, because ‘One and the same [Person] is . . . truly the Son of God and truly the Son of Man’ (Ep. xxviii, 4 PL. liv, 767). Whence ‘In his actions either nature with the co-operation of the other performs what is proper to it; thus the Word performs the part of the Word, and the humanity the part of the humanity’ (Ibid). In these expressions appears the use of what is called the Common Application of Terms (Communicatio Idiomatum), which Cyril vindicated against Nestorius. It depends on the firm foundation that both natures subsist by the One Person of the Word begotten before all ages of the Father and born of Mary according to the flesh in the course of time.
This sublime doctrine, which is drawn from the gospels and differs in no way from that of the council of Ephesus refutes Eutyches as well as Nestorius. The dogmatic definition of the council of Chalcedon concords with it absolutely and perfectly, for this definition likewise defines two distinct natures and one person in Christ in the following clear and precise words: ‘This great and holy oecumenical council condemns those who pretend that there were two natures in the Lord before the union, and imagine that there was only one after the union. Following, therefore, in the traditions of the holy Fathers we teach that all with one voice confess that the Son [of God] and our Lord Jesus Christ are one and the same, and that he is perfect in his divinity, perfect in his humanity, true God and true man, made of a rational soul and a body, consubstantial with the Father in his divinity, and the same also in his humanity received from the Virgin Mary in recent times for our sake and for our salvation, one and the same Christ, the Son, the Lord, the Only Begotten, having two natures without confusion, change, division or separation; the distinction between the natures was not removed by the union, but the properties of each remain inviolate and are joined together in one person. He is not sundered or divided into two persons, but is one and the same Son and only Begotten God the Word, the Lord, Jesus Christ’ (Mansi. vii, 114 and 115).
If anyone asks how it is that the statements of the council of Chalcedon are of such outstanding excellence in their clarity and their efficiency in the refutation of error, we reply that this arises from the fact that ambiguities had been removed and a most exact terminology was used. For in the Chalcedonian definition of the faith and the same concept underlies the terms ‘Person’ (Prósopon) and ‘Hypostasis’ (Upóstasis); the term ‘Nature’ has a totally different sense, and its meaning is never given to the other words. So that the Nestorians and Eutychians of old and certain modern writers err when they maintain that the council of Chalcedon corrected the decision of the council of Ephesus. Rather the one perfected the other, so that a synthesis or composition of the main Christological doctrine was available in fuller form for the second and third oecumenical councils of Constantinople.
It is indeed sad that the ancient adversaries of the council of Chalcedon (also called Monophysites) should have rejected this doctrine, so lucid, so coherent and so complete, on the strength of certain badly understood expressions of ancient writers. While they rejected the absurd teaching of Eutyches about the mixture of natures in Christ, they obstinately clung to the well-known expression: ‘One Incarnate nature of the Word God’. This expression had been used by Cyril of Alexandria (who took it from St. Athanasius) with a perfectly correct meaning, since he used the term ‘nature’ to signify ‘person’. The Fathers of Chalcedon, therefore, totally removed what was ambiguous or liable to cause error in these expressions. For they applied the same terms as are used in the theology of the Trinity, to the exposition of our Lord’s Incarnation. Thus they made ‘nature’ and ‘essence’ (essentia) the same, and likewise ‘Person’ and ‘Hypostasis’, and they treated the latter two names as totally different in meaning, from the former two. Their approach, on the other hand, had made ‘nature’ the equivalent of’ Person’ not of ‘essence’ (essentia). –from Sempiternus Rex Christus (1951)