Chapter 13.
He explains who are those in whose person the Prophet Isaiah says: “You are our God, and we knew You not.”
“You are then,” he says, “our God, and we knew You not, O God of Israel the Saviour.” Who do you imagine chiefly say this; and in whose mouths are such words specially suitable, Jews or Gentiles? If you say Jews: certainly the Jews did not know Christ, as it is said, “But Israel has not known Me, My people have not considered;” and, “The world was made by Him, and the world knew Him not. He came unto His own, and His own received Him not.” But if you say Gentiles, it is clear that the Gentile world was given over to idols, and knew not Christ, though it knew not the Father any more; but still if it has now come to know Him, it is only through Christ. You see then that whether the believing people belong to the Jews or the Gentiles, in either case they can truly say for themselves: “You are our God; and we knew You not, O God of Israel the Saviour.” For the Gentiles who formerly worshipped idols knew not God; and the Jews who denied the Lord, knew not the Son of God. And thus both truly say of Christ: “You are our God and we knew You not.” For those who did not believe in God were as ignorant of Him as those who denied the Son of God. If therefore Christ is to be believed in, as the truth declares, as the Deity asserts, as indeed Christ Himself declares, who is both, why are you miserably trying in your madness to interpose between God and Christ? Why do you seek to divide His body from the Son of God, and try to separate God from Himself? You are severing what is one, and dividing what is joined together. Believe the Word of God concerning God: for you cannot possibly make a better confession of God's Divinity than by confessing with your voice that which God teaches about Himself. For you must know that, as the Prophet says, “the Lord Himself is God, who found out all the way of knowledge; who was seen upon earth and conversed with men.” He brought the light of faith into the world. He showed the light of salvation. “For God is the Lord, and has given us light.” Then believe Him, and love Him, and confess Him. For since, as it is written, “Every knee shall bow to Him, of things in heaven, and things on earth, and things under the earth, and every tongue shall confess that Jesus Christ is Lord in the glory of God the Father,” whether you will or no, you cannot deny that Jesus Christ is Lord in the glory of God the Father. For this is the crowning virtue of a perfect confession, to acknowledge that Jesus Christ is ever Lord and God in the glory of God the Father.
Source: On the Incarnation of the Lord (Against Nestorius) (New Advent)