Cyril opens his five books against Nestorius by stating plainly that he would have preferred silence. The teacher he is about to refute holds the most prominent see in the Eastern empire, and the consequences of public disputation in such a case are visible to anyone who can read the times. Yet Cyril writes, because what is being denied is not a private opinion but the rule of faith. If the Christ who is preached is not the one Lord confessed by Nicaea, no preaching at all will save.

He sets out his method at the outset. He will quote Nestorius in his own words, drawn from sermons and writings already published, and answer them line by line. He will not impute meanings that Nestorius has not stated. His readers can verify the citations against the sermons that are circulating; what he is going to refute is what Nestorius actually says.

Cyril takes up Nestorius's central move: the division of the one Lord Jesus Christ into two distinct subjects, one divine and one human, joined by a moral or relational union rather than a hypostatic one. Cyril answers from Scripture. The same Jesus who weeps at the tomb of Lazarus is the one who calls Lazarus out of it. The same Christ who is hungry in the wilderness is the one who feeds five thousand on a hillside. The Gospel does not parcel out its verses between two subjects; it gives them all to one Person.

Cyril names what Nestorius's division actually produces: not one incarnate Son, but a man indwelt by the Word as one of the prophets was, and a Word who only seems to suffer because a man near to Him is suffering. Such a Christ saves no one, because His sufferings are not the sufferings of God for us.

The pastoral occasion of the controversy was Nestorius's refusal to allow the Virgin Mary to be called Theotokos, the God-bearer. Cyril answers that the title is older than the controversy. It was sung in the prayers of the Church before Nestorius was born. It expresses what the apostolic Gospel says plainly: the one whom Mary bore is God the Word made flesh.

Cyril is careful with the word. To call Mary Theotokos is not to say that her flesh contributed something to the divine nature, as if the Word received existence or essence from her. It is to confess that the One whom she carried for nine months and laid in the manger is the eternal Son. He took His humanity from her; He did not take His divinity from her. The title guards the personal unity of Christ. To refuse it is to confess two Christs.

Cyril answers a further Nestorian move: that the divine and human natures of Christ have distinct operations, the Word doing the miracles and the man doing the suffering, by a kind of cooperation between two persons. Cyril answers that Scripture witnesses only to one operation belonging to one Person, who acts divinely through His flesh and humanly through the divinity that has united flesh to itself.

When Christ touches the leper and the leprosy departs, it is not the Word touching at a distance through a man's hand; it is the incarnate Word, touching with His own hand, healing with His own divine power, which is now communicated to the body that is His. The fire of divinity warms the iron of humanity; the iron, while remaining iron, glows now with the heat that belongs to fire.

Cyril ends the work by drawing his Christology into the sacramental life of the Church. If Nestorius is right and the Word is not personally united to the flesh, then the flesh that is given to the faithful in the Eucharist is the flesh of a man with whom the Word is associated, not the flesh that is His own. The bread on the altar gives a creature, not the life-giving body of the incarnate Lord.

Cyril answers that the Eucharist is itself the rule. The Church receives, week by week, the body that the Word has made His own; it receives the flesh that is the flesh of God; it receives the medicine of immortality. The doctrine that denies the personal union destroys the sacrament. The doctrine that confesses it makes sense of what the Church has always done at the altar.