22 But as He by His own act assumed a body from the Virgin, so He assumed from Himself a soul; though even in ordinary human birth the soul is never derived from the parents. If, then, the Virgin received from God alone the flesh which she conceived, far more certain is it that the soul of that body can have come from God alone. If, too, the same Christ be the Son of Man, Who is also the Son of God (for the whole Son of Man is the whole Son of God), how ridiculous is it to preach besides the Son of God, the Word made flesh, another I know not whom, inspired, like a prophet, by God the Word; whereas our Lord Jesus Christ is both Son of Man and Son of God.
Yet because His soul was sorrowful unto death, and because He had the power to lay down His soul and the power to take it up again, they want to derive it from some alien source, and not from the Holy Ghost, the Author of His body's conception: for God the Word became man without departing from the mystery of His own nature. He was born also not to be at one time two separate beings, but that it might be made plain, that He Who was God before He was Man, now that He has taken humanity, is God and Man.
How could Jesus Christ, the Son of God, have been born of Mary, except by the Word becoming flesh: that is by the Son of God, though in the form of God, taking the form of a slave? When He Who was in the form of God took the form of a slave, two contraries were brought together. Thus it was just as true, that He received the form of a slave, as that He remained in the form of God. The use of the one word 'form' to describe both natures compels us to recognise that He truly possessed both.
He is in the form of a servant, Who is also in the form of God. And though He is the latter by His eternal nature, and the former in accordance with the divine Plan of Grace, the word has its true significance equally in both cases, because He is both: as truly in the form of God as in the form of Man. Just as to take the form of a servant is none other than to be born a man, so to be in the form of God is none other than to be God: and we confess Him as one and the same Person, not by loss of the Godhead, but by assumption of the manhood: in the form of God through His divine nature, in the form of man from His conception by the Holy Ghost, being found in fashion as a man.
That is why after His birth as Jesus Christ, His suffering, death, and burial, He also rose again. We cannot separate Him from Himself in all these diverse mysteries, so that He should be no longer Christ; for Christ, Who took the form of a servant, was none other than He Who was in the form of God: He Who died was the same as He Who was born: He Who rose again as He Who died; He Who is in heaven as He Who rose again; lastly, He Who is in heaven as He Who before descended from heaven.
Source: On the Trinity (New Advent)