Chapter 34 is one of three extra-long chapters in Book IV of St. Thomas’ SCG. Here he tackles Nestorianism.
First he gives a “steel man” outline of the heresy. Steel man as opposed to straw man. St. Thomas always gives his opponents’ arguments as much coherence as possible.
St. Thomas explains what Nestorianism tried to prevent: We should not attribute to God things that are incompatible with the divine nature, like suffering and dying.
Then St. Thomas points out the fundamental Scriptural problem with Nestorianism. He makes grammatical arguments against it. Then he carefully considers important New-Testament passages that touch on the problem.
Please keep in mind the unusual but crucially important term we considered when we read Chapter 5. A supposit is a particular instance of a type of thing. When the type of thing is “rational being,” St. Thomas also uses the Greek term hypostasis. Both supposit and hypostasis appear repeatedly in Chapter 34, since we are focusing on the particular rational being, the particular person, Jesus.
Chapter 34 leaves us with…
a. The Blessed Virgin Mary truly is the Mother of God.
b. The magnificent mystery of the “communication of idioms.” That is, everything about the Christ, all the divine things and all the human things, can be ascribed to the one Person who is Jesus.
For example:
We can say–we must say–“God died.” Not because God died in His divine nature, but because He died in His human nature, which He assumed to His divine Self in the Incarnation.
And we can say “Jesus created the universe,” not because Jesus the man created the universe (Jesus the man is a creature) but because Jesus is, as a Person, the eternal Word, the Creator.
The Christian religion sits on this fundamental bedrock: the unity of the Person of the God-man, Jesus of Nazareth. In other words, we Christians, beginning with the Apostles, believe in the Incarnation.
Father Mark White Blog 