The Seven Books of John Cassian on the Incarnation of the Lord, Against Nestorius 21He shows once more by other passages of the Apostle that Christ is God.

I want you to tell me, you heretic, whether in this passage He who, as the Apostle tells us, speaks in him, is man or God. If He is man, how can another's body speak in his heart? If God, then Christ is not a man but God; for since Christ spoke in the Apostle, and only God could speak in him, therefore a Divine Christ spoke in him. And so you see that there is nothing to be said here, that no division or separation can be made between Christ and God: because complete Divinity was in Christ, and Christ was completely in God. No division or severing of the two can here be admitted. There is only one simple, pious, and sound confession to be made; viz., to adore, love, and worship Christ as God. But do you want to understand more fully and thoroughly that there is no separation to be made between God and Christ, and that we must hold that God is altogether one with Christ? Hear what the Apostle says to the Corinthians: "For we must all be manifested before the judgment seat of Christ, that every one may receive the proper things of the body, according as he hath done, whether it be good or evil." But in another passage, in writing to the Romans he says: "We shall all stand before the judgment seat of God: for it is written: As I live, saith the Lord, every knee shall bow to me, and every tongue shall confess to God." You see then that the judgment seat of God is the same as that of Christ; understand then without any doubt that Christ is God; and when you see that the substance of God and Christ is altogether inseparable, admit also that the Person cannot be severed. Unless forsooth because the Apostle in one Epistle said that we should be manifested before the judgment seat of Christ, and in another before that of God, you invent two judgment seats, and fancy that some will be judged by Christ and others by God. But this is foolish and wild, and madder than a madman's utterances. Acknowledge then the Lord of all, the God of the universe, acknowledge the judgment seat of God in the judgment seat of Christ. Love life, love your salvation, love Him by whom you were created. Fear Him by whom you are to be judged. For whether you will or no, you have to be manifested before the judgment seat of Christ, and laying aside wicked blasphemy and the childish talk of unbelieving words, though you think that the judgment seat of God is different from that of Christ, you will come before the judgment seat of Christ, and will find by evidence that there is no gainsaying, that the judgment seat of God is indeed the same as that of Christ, and that in Christ the Son of God, there is all the glory of God the Son, and the power of God the Father. "For the Father judgeth no man, but hath committed all judgment to the Son, that all men may honour the Son as they honour the Father." For whoever denies the Father denies the Son also. "Whosoever denieth the Son, the same hath not the Father: he that confesseth the Son, hath the Father also." And so you should learn that the glory of the Father and the Son is inseparable, and their majesty is inseparable also and that the Son cannot be honoured without the Father, nor the Father without the Son. But no man can honour God and the Son of God except in Christ the only-begotten Son of God. For it is impossible for a man to have the Spirit of God who is to be honoured except in the Spirit of Christ, as the Apostle says: "But ye are not in the flesh, but in the Spirit, if so be that the Spirit of God dwell in you. But if any man have not the Spirit of Christ, he is none of His." And again: "Who shall lay anything to the charge of God's elect? It is God that justifieth. Who is he that condemneth? It is Christ Jesus who died, yea rather who rose again." You see then now, even against your will, that there is absolutely no difference between the Spirit of God and the Spirit of Christ, or between the judgment of God and the judgment of Christ. Choose then which you will--for one of the two must happen--either acknowledge in faith that Christ is God, or admit that God is in Christ at your condemnation.