Introduction
Having just read St. Augustine's wonderful work De Trinitate (which can be bought here) I was struck by how closely aligned St. Augustine and St. Cyril's Christologies are. It has been popular to label St. Augustine a "Crypto-Nestorian," but a simple reading of St. Augustine (NOT a catena of out of context quotes which you found on the internet, for, laziness becomes heresy) shows this to be a ridiculous reading of St. Augustine up there with Trail of Blood. For the background to this four-aspect Christology of St. Cyril, see the article where I develop it more fully here.
Unity of Person
The first tenant of Cyrillian Christology is that of the Unity of person, against Nestorious who created “a different Son and Christ and Lord.” (On the Unity of Christ). St. Augustine affirms this unity of Person, writing that the eternal Logos took upon Himself “the form of man” (De Trinitate 1.7).
Single-Subjectivity
The second tenant of Cyrillian Christology is that of single subjectivity, that, even though we may speak of a double predication of certain actions and attributes, we must speak of these in regard to one subject, the person of the Logos. St. Cyril fights against Nestorious who “split up and completely divide his words and acts.” (On the Unity of Christ). St. Augustine follows this, echoing Paul, by saying “the Lord of glory was crucified.” (De Trinitate 1.13) St. Augustine quotes this verse to prove that it was the single subject of God who suffered and died.
Two True Natures
The third tenant of Cyrillian Christology is that, although there is a single person and single subject, there are two full and true natures in the Logos, Divinity and Humanity. St. Cyril acknowledges that in the incarnation the one who was “God by nature” took on “flesh,” (On the Unity of Christ), yet this did not cause a “negation of the difference of natures…[rather] the Godhead and the manhood by their ineffable and indescribable consilience into unity achieved One Lord and Christ and Son for us.” (Second Letter to Nestorius) St. Augustine too affirms this, writing, “the very same [is] the Son of man on account of the form of a servant which He took, who is the Son of God on account of the form of God in which He is.” (De Trinitate 1.13).
Dual Predication
Lastly, the fourth tenant of Cyrillian Christology is that of Dual Predication, i.e. the Communicatio Idiomatum, that we may predicate certain actions and attributes according to each of the natures, yet of the single person. This is really the center of St. Cyril’s Christology. He insisted on the term Theotokos because of the Unity of person, and explained how God could have a mother, using Dual Predication, i.e. in the words of the Chalcedonian definition “born of the Virgin Mary, the Mother of God, according to the Manhood.” St. Augustine follows the same pattern, he often speaks in De Trinitate of Christ doing a certain action according to “the form of God” or “the form of man.” For example, “He was crucified after the form of a servant [dual predication], and yet the Lord of Glory was crucified [single subjectivity].” This language of “forms” to describe this doctrine was one used in the west, most famously (or infamously) by the Tome of Pope St. Leo, writing “Accordingly He who while remaining in the form of God made man, was also made man in the form of a slave.” This language is the exact same language which is used by St. Augustine and it is thought by some that Pope St. Leo is here drawing from St. Augustine for his Tome which is accepted at the Council of Chalcedon as being an orthodox expression.
Conclusion
In Conclusion, although we may quibble that St. Augustine lacks the precision of later authors such as Pope St. Leo, or St. Cyril in his language, we may appreciate the rock-solid orthodoxy of it. We must give the judgment of charity to him (in opposition to those who accuse the Doctor Gratiae of Nestorianism), and recognize that St. Augustine lived in a time before the Councils of Ephesus and Chalcedon, and before the St. Cyril/Nestorius debate had been too widely promulgated.