I recently finished reading John Cassian’s work On the Incarnation of the Word of God, Against Nestorius (De Inc.). This work was written in reaction to certain documents of Nestorius’ circulating in the West. Cassian wrote it at the request of Archdeacon Leo of Rome (later Pope Leo). George Bevan sees evidence of Cassian working here with four sermons of Nestorius’ and the Contestatio of Eusebius — “Slender evidence” (116).*
The result of Cassian’s slender evidence is a document that won’t do you much good if you want to learn about Nestorius and the arguments against his teaching. As well, Cassian is not always as precise with his language as fifth-century theologians ought to be, and some of his sentences and phrases could have been misconstrued. However, overall, the orthodoxy of the document is that Christ is one person who has two natures, and Cassian argues against certain terminology that could be used to deny this reality.
Cassian takes the reader through a discussion of one heretic’s recantation, and then a refutation of the Nestorian documents at his disposal using Scripture, then using the baptismal creed of Antioch, and then, in Book 7, the Fathers — Sts. Hilary of Poitiers, Ambrose of Milan, Jerome, Rufinus of Aquileia, Augustine of Hippo, Gregory of Nazianzus, Athanasius, and John Chrysostom.
This document illustrates, especially in 6.5, the inherent conservatism and traditionalism of ancient Christianity. We also see why any truly orthodox Christian can comfortably call Mary Theotokos, or “God-bearer” (usually rendered “Mother of God”).
I, personally, like this book for some of Cassian’s statements that seem (to me, at least) pretty clear statements of orthodoxy:
Therefore the Lord Jesus Christ is God. But if He be, as He certainly is, God: then she who bore God is Theotocos, i.e., the mother of God. Unless perhaps you want to take refuge in so utterly absurd and blasphemous a contradiction as to deny that she from whom God was born is the mother of God, while you cannot deny that He who was born is God. (2.5)
God could not possibly be known of men, unless He Himself gave us the knowledge of Himself. (4.2)
And so following the guidance of the sacred word we may now say fearlessly and unhesitatingly that the Son of man came down from heaven, and that the Lord of Glory was crucified: because in virtue of the mystery of the Incarnation, the Son of God became Son of man, and the Lord of Glory was crucified in (the nature of) the Son of man. What more is there need of? It would take too long to go into details: for time would fail me, were I to try to examine and explain everything which could be brought to bear on this subject. For one who wished to do this would have to study and read the whole Bible. For what is there which does not bear on this, when all Scripture was written with reference to this? (4.7)
He then alone it is who spake to the patriarchs, dwelt in the prophets, was conceived by the Spirit, born of the Virgin Mary, appeared in the world, lived among men, fastened to the wood of the cross the handwriting of our offences, triumphed in Himself, slew by His death the powers that were at enmity and hostile to us; and gave to all men belief in the resurrection, and by the glory of His body put an end to the corruption of man’s flesh. You see then that all these belong to the Lord Jesus Christ alone: and therefore no other shall be accounted of in comparison with Him, for He alone is God begotten of God in this glory and unique blessedness. (4.9)
For all who believe in God are sons of God by adoption: but the only begotten alone is Son by nature: who was begotten of His Father, not of any material substance, for all things, and the substance of all things exist through the only begotten Son of God—and not out of nothing, because He is from the Father: not like a birth, for there is nothing in God that is void or mutable, but in an ineffable and incomprehensible manner God the Father, wherein He Himself was regenerate, begat his only begotten Son; and so from the Most High, Ingenerate, and Eternal Father proceeds the Most High, Only Begotten, and Eternal Son. (5.4)
There is nothing wanting then in the Creed; because as it was formed from the Scriptures of god by the apostles of God, it has in it all the authority it can possibly have, whether of men or of God. (6.4)
This is our faith; this is our salvation: to believe that our God and Lord Jesus Christ is one and the same before all things and after all things. (6.19)
You can read it for yourself at the CCEL.
*The Case of Nestorius: Ecclesiastical Politics in the East 428-451 CE.