7 Well, this consideration indeed is able to school us in moral character, but it is necessary also to state the cause for which those women were barren. What then was the cause? It was in order that when you have seen the Virgin bringing forth our common Master, you might not disbelieve. Wherefore exercise your mind in the womb of the barren; in order that when you have seen the womb, disabled and bound as it is, being opened to the bearing of children from the grace of God, you might not marvel at hearing that a virgin has brought forth. Or rather even marvel and be astounded; but do not disbelieve the marvel. When the Jew says to you, “how did the virgin bear?” say to him “how did she bear who was barren and enfeebled by old age?” There were then two hindrances, both the unseasonableness of her age and the unserviceableness of nature; but in the case of the Virgin there was one hindrance only, the not having shared in marriage. The barren one therefore prepares the way for the virgin. And that you may learn that it was on this account that the barren ones had anticipated it, in order that the Virgin's childbirth might be believed, hear the words of Gabriel which were addressed to her— For when he had come and said to her, “you shall conceive in the womb and bear a son, and you shall call his name Jesus;” the Virgin was astonished and marvelled, and said, “how will this be to me, since I know not a man.” What then said the Angel? “The Holy Ghost shall come upon you.” Seek not the sequence of nature, he says, when that which takes place is above nature; look not round for marriage and throes of child-birth, when the manner of the birth is too grand for marriage. “And how will this be,” she says, “since I know not a husband.” And verily on this account shall this be, since you know no husband. For did you know a husband, you would not have been deemed worthy to serve this ministry. So that, for the reason why you disbelieve, for this believe. And you would not have been deemed worthy to serve this ministry, not because marriage is an evil; but because virginity is superior; and right it was that the entry of the Master should be more august than ours; for it was royal, and the king enters through one more august. It was necessary that He should both share as to birth, and be diverse from ours. Wherefore both these things are managed.
For the being born from the womb is common in respect to us, but the being born without marriage is a thing greater than on a level with us. And the gestation and conception in the belly belongs to human nature; but that the pregnancy should take place without sexual intercourse is too august for human nature. And for this purpose both these things took place, in order that you may learn both the pre-eminence and the fellowship with you of Him who was born.
Source: Homily Against Publishing the Errors of the Brethren (New Advent)