Irenaeus of Lyons · a new plain-English translation from the original language
What is meant by what was said to David: Of the fruit of your womb I will set upon my throne.
And in saying, Hear, O house of David, it was indicating that the one whom God pledged to David that he would set up as an everlasting King out of the fruit of her womb is the same one born of the Virgin descended from David's line. God made this promise concerning a King from the fruit of the womb precisely because that belongs to a pregnant virgin, and not from the fruit of the loins or of the kidneys, which belongs to a man who fathers and a woman who conceives through a man. Scripture, accordingly, bypassed the generative organs of a man entirely in this promise — it does not so much as mention them — since he who was to be born did not arise through a man's will. Instead it set down and made certain the fruit of the womb, thereby announcing the birth of him who was to come from a virgin, just as Elizabeth, having been filled with the Holy Spirit, declared to Mary: Among women you are blessed, and blessed too is the offspring within your womb — the Holy Spirit thereby making known to any willing to listen that the promise God had made, to raise a King from the fruit of her womb, was accomplished in the Virgin's childbearing, that is, in Mary. Those, then, who distort what Isaiah says — that a young woman would carry a child in her womb — and insist that Joseph was his father, why do they not likewise distort the promise recorded concerning David, where God pledged to set up out of the fruit of his womb the horn of the King, Christ? But this they failed to grasp; otherwise they would have ventured to twist this passage too.
And regarding Isaiah's phrase, in the depths beneath, or in the heights above, this pointed to the fact that the one who went down was the very same one who also went up. But when it says, The Lord himself will grant a sign in the house of David — what remarkable thing, what sign at all, would that be if a young woman conceived by a man and bore a child, as happens with every woman who bears children in the ordinary way? Rather, because an unlooked-for salvation was drawing near for humankind through God's own working, an unlooked-for childbearing by a Virgin likewise came to pass, God himself furnishing this as the sign, not any human agency bringing it about.