Nicene and Post-Nicene Fathers: Series II/Volume XI/John Cassian/Against Nestorius/Book IV/Chapter 5

Chapter V.

How in virtue of the hypostatic union of the two natures in Christ the Word is rightly termed the Saviour, or incarnate man, and the Son of God.

so it is clear that through the mystery of the Word of God joined to man, the Word, which was sent to save men, can be termed Saviour, and the Saviour, who was born in the flesh, can through union with the Word be called the Son of God; and so through the indifferent use of either title, since God is joined to man, whatever is God and man, can be termed altogether God. And so the same Apostle well adds the words: &#8220;Whoever believeth that Jesus is the Son of God, God abideth in him, and the love of God is perfected in him.&#8221; He tells us that he believes, and declares that he is filled with divine love, who believes that Jesus is the Son of God. But he testifies that the Word of God is the Son of God, and thus means us fully to understand that the only begotten Word of God, and Jesus Christ the Son of God are one and the same Person. But do you want to be told more fully that,&#8212;though Christ according to the flesh was truly born as man of man,&#8212;yet in virtue of the ineffable unity of the mystery, by which man was joined to God, there is no separation between Christ and the Word? Hear the gospel of the Lord, or rather hear the Lord

Himself saying of Himself: &#8220;This,&#8221; says He, &#8220;is life eternal, that they may know Thee, the only true God, and Jesus Christ whom Thou hast sent.&#8221; You heard above that the Word of God was sent to heal mankind: here you are told that He who was sent is Jesus Christ. Separate this, if you can,&#8212;though you see that so great is the unity of Christ and the Word, that it was not merely that Christ was united with the Word, but that in virtue of the actual unity [of Person] Christ may even be said to be the Word.