Nicene and Post-Nicene Fathers, Ser. II, Vol. XI:
The Works of John Cassian.: Chapter XX. He shows from what has been said that we do not mean that God was mortal or of flesh before the worlds, although Christ, who is God from eternity and was made man in time, is but one Person.

Chapter XX.

He shows from what has been said that we do not mean that God was mortal or of flesh before the worlds, although Christ, who is God from eternity and was made man in time, is but one Person.

But perhaps you will say: If I admit that the same Person was in the end of time born of a Virgin, who was begotten before all things of God the Father, I shall imply that before the beginning of the world God was in the flesh, as I say that He was afterwards man, who was always God: and so I shall say that that man who was afterwards born, had always existed. I do not want you to be confused by this blind ignorance, and these obscure misconceptions, so as to fancy that I am maintaining that the manhood 2578 which was born of Mary had existed before the beginning of things, or asserting that God was always in a bodily form before the commencement of the world.  I do not say, I repeat it, I do not say that the manhood was in God before it was born: but that God was afterwards born in the manhood. For that flesh which was born of the flesh of the Virgin had not always existed: but God who always was, came in the flesh of man of the flesh of the Virgin. For “the Word was made flesh,” and did not manifest flesh together with Himself: but in the glory of Divinity joined Himself to human flesh. For tell me when or where the Word was made flesh, or where He emptied Himself by taking the form of a servant: or where He became poor, though He was rich? Where but in the holy womb of the Virgin, where at His Incarnation, the Word of God is said to have been made flesh, at His birth He truly took the form of a servant; and when He is in human nature nailed to the Cross, He became poor, and was made poor in His sufferings in the flesh, though He was rich in His Divine glory? Otherwise if, as you say, at some later period the Deity entered into Him as into one of the Prophets and saints, then “the Word was made flesh” in those men also in whom He vouchsafed to dwell: then in each one of them He emptied Himself and took upon Him the form of a servant. And thus there is nothing new or unique in Christ. Neither His conception, nor His birth nor His death had anything special or miraculous about it.

Footnotes

Hominem.