The following reflection on the virgin birth is excerpted from Karl Barth, Church dogmatics I/2, London; New York: T&T Clark. p.172.
In the [virgin birth of Christ] there is contained a judgment upon man. When Mary as a virgin becomes the mother of the Lord and so, as it were, the entrance gate of divine revelation into the world of man, it is declared that in any other way, i.e., by the natural way in which a human wife becomes a mother, there can be no motherhood of the Lord and so no such entrance gate of revelation into our world. In other words, human nature possesses no capacity for becoming the human nature of Jesus Christ, the place of divine revelation. It cannot be the work-mate of God. If it actually becomes so, it is not because of any attributes which it possessed already and in itself, but because of what is done to it by the divine Word, and so not because of what it has to do or give, but because of what it has to suffer and receive—and at the hand of God.
The virginity of Mary in the birth of the Lord is the denial, not of man in the presence of God, but of any power, attribute or capacity in him for God. If he has this power—and Mary clearly has it—it means strictly and exclusively that he acquires it, that it is laid upon him. In this power of his for God he can as little understand himself as Mary in the story of the Annunciation could understand herself as the future mother of the Messiah. Only with her [“behold the hand-maid of the Lord”] can he understand himself as what, in a way inconceivable to himself, he has actually become in the sight of God and by His agency.
The meaning of this judgment, this negation, is not the difference between God as Creator and man as a creature. Man as a creature—if we try for a moment to speak of man in this abstract way—might have the capacity for God and even be able to understand himself in this capacity. In Paradise there would have been no need of the sign [of the virgin birth] to indicate that man was God’s fellow-worker. But the man whom revelation reaches, and who is reconciled to God in revelation and by it, is not man in Paradise. He has not ceased to be God’s creature. But he has lost his pure creatureliness, and with it the capacity for God, because as a creature and in the totality of his creatureliness he became disobedient to his Creator. To the roots of his being he lives in this disobedience.
It is with this disobedient creature that God has to do in His revelation. It is his nature, his flesh, that the Word assumes in being made flesh. And this human nature, the only one we know and the only one there actually is, has of itself no capacity for being adopted by God’s Word into unity with Himself, i.e., into personal unity with God. Upon this human nature a mystery must be wrought in order that this may be made possible. And this mystery must consist in its receiving the capacity for God which it does not possess. This mystery is signified by the [virgin birth].