We read Cyril's On the Unity of Christ during my second semester at college. It blew my mind. Let me tell you a little about it.
It starts off with a statement that was true when he wrote it, and remains true today: "People of true and good sense, who have intellectually gathered that knowledge which gives life, are never jaded by the sacred sciences. Indeed it is written that 'Man shall not live by bread alone, but by every word that comes from the mouth of God.' The word of God is food for the mind and a spiritual 'bread that strengthens the heart of man,' as the Book of Psalms sings (Ps. 104:15)."
He centers his discussion on one thing: The word of God. That is where the discussion must start, and no matter where the discussion goes, it must end there as well.
Cyril wrote this treatise because of a heresy that was gaining ground in the church. Nestorius (a bishop) had denied that Mary, mother of Jesus, should be called theotokos: "Mother of God." Instead, he favored "Mother of Christ," seeing the Christ as a sort of team-up between God and Man: The eternal, divine Son of God, entering into a "conjunction" with the man Jesus, which, in turn, would mean that the man Jesus was never actually God.
This is, as Cyril points out, a fairly large problem. He goes on to explain this problem and the solution in Christian Orthodoxy in great detail, but not without getting in a few good jabs along the way. Which brings me to...
The structure of the argument
What first struck me about this book, before I got to any of the awesome theology, was the style of argumentation. It's structured as a "conversation" between Speaker A and Speaker B. Speaker A is the master (presumably Cyril himself), while Speaker B is the hapless apprentice, unable to determine between orthodoxy and heresy. A typical exchange goes something like this:
Speaker B: Hey Master, those guys say this about Jesus and the Word. What do you think?
Speaker A: [Sighs, closes eyes and rubs temples] I'm going to level with you, buddy: If a nation composed entirely of idiots decided to find the five biggest idiots in the entire country, and fused those idiots together to form Idiot Voltron, and then Idiot Voltron said the absolute dumbest thing he could think of, it still would not be anywhere close to being as dumb as what you just said. Seriously, it is the worst."
And so on. Actual quotes include, "Away with such a horrid and vile opinion! These are the teachings of a wanderer, of a sick mind," and "My goodness. I cannot imagine how stupid and intellectually superficial they must be who hold to such a conception."
It's a wonderful style of argument: it consists in a 50-50 split between claiming that his opponents are the kind of idiots that idiots look down on, and absolutely fantastic theology.
Absolutely Fantastic Theology
This book changed everything about everything. My mind was blown several times reading this book (as evidenced by the near frantic underlining and highlighting that mars about 3/4's of the overall book). This happened for two reasons:
1: The whole issue of the Incarnation, and what it means for God to be man while still being God, is way more complicated than I had ever imagined. And...
2: Cyril does a freaking awesome job of explaining it anyway.
A "summary" would do this book an incredible injustice. Cyril pulls from Scripture after Scripture to demonstrate that the Christ was no mere "conjunction" between God and man. Indeed, such a conjunction would do us no good at all, for the birth and death of Jesus would have been nothing more than the birth and death of a man.
No: For us to be saved, the Christ must be something more than that. Cyril sums up this "something more" incredibly well: "Godhead is one thing, and manhood is another thing, considered in the perspective of their respective and intrinsic beings, but in the case of Christ they came together in a mysterious and incomprehensible union without confusion or change."
And this union is, as he says, "entirely beyond conception." In fact, in describing it, Cyril at times almost seems to agree with those who would split the Christ into two: Saying things like "He remained Lord of all things even when he came in the form of a slave", and "Both he who exists in lordly glories, and he who took the form of a slave as his own, calls God his Father." But these phrases are, in fact, expressions of a much deeper and more mysterious truth: That of an eternal and infinite God entering a finite and temporal world. It's not going to be neat. It's not going to be understandable. There's always going to be something to wonder about, because our minds are finite and cannot understand infinity.
And Cyril, despite his love of theology (the "sacred sciences")--or rather, because of his love of theology--marvels at that mystery. He loves it, and he never loses that crucial sense of awe and wonder. He remains forever conscious that what he is discussing is no mere academic phenomenon, no theoretical problem to be worked out, but something that happened, the happening of which is indescribably important. That's why he often ends particularly complicated or technical passages with phrases like "And that is why the mystery of Christ is truly wonderful."
And he ends it with a passage to top everything off: Here it is.
This is why we believe that there is only one Son of God the Father. This is why we must understand Our Lord Jesus Christ in one person. As the Word he is born divinely before all ages and times, but in these last times of this age the same one was born of a woman according to the flesh. To the same one we attribute both the divine and human characteristics, and we also say that to the same one belongs the birth and the suffering on the cross, since he appropriated everything that belonged to his own flesh, while ever remaining impassible in the nature of the Godhead. This is why 'every knee shall bend before him, and every tongue shall confess that Jesus Christ is Lord, to the glory of God the Father."
Amen.This book forever changed the way I think about the Incarnation. It was the subject of two of my Torrey papers (one embarrassingly bad) and figured heavily in a third paper, which I am still proud of. I would go on to read many, many books on the Incarnation and the Word, all the way from Augustine's The Trinity to Barth's The Way of the Son of God to The (absolutely horrible) Shack, and Cyril's Unity impacted my reading of every single one. It is the epitome of Incarnational orthodoxy, and you should definitely read it.
Btw, here's an Amazon link to the edition I have: it's got a nice preface that includes the historical and theological background for all of this. Very cool.
1: The Man Who Was Thursday by G.K. Chesterton
2: (Everything else by Chesterton: Manalive, Orthodoxy, The Ball and the Cross)
3: On the Unity of Christ by Cyril of Alexandria
4: The Way of the Son of God into the Far Country by Karl Barth
5: The Lord of the Rings by JRR Tolkien
6: CS Lewis section (Space Trilogy, Chron. of Narnia, Abolition of Man, Till We Have Faces)
7: The book of Job
8: The book of Ecclesiastes
9: The Idea of a University by John Henry Newman
10: The Thief, by Megan Whalen Turner