8 And pray consider the wisdom of all that was done. Neither did the pre-eminence injure the likeness and kinship to us, nor did the kinship to us dim the pre-eminence; but both were displayed by all the circumstances; and the one had our condition in its entirety, and the other what was diverse compared with us. But just as I was saying, on this account the barren ones went before, in order that the Virgin's child-birth might be believed, that she might be led by the hand to faith in that promise and undertaking which she heard from the angel, saying, “The Holy Ghost shall come upon you, and the miraculous power of the Most High shall overshadow you”— thus, he says, you are able to bear.
Look not to the earth; it is from the heavens that the operation will come. That which takes place is a grace of the Spirit; pray inquire not about nature and laws of marriage. But since those words were too high for her, he wills to afford also another demonstration. But do thou, pray, observe how the barren one leads her on the way to the belief in this. For since that demonstration was too high for the Virgin's intelligence, hear how he brought down what he said to lower things also, leading her by the hand by sensible facts.
For “behold,” he says, “Elizabeth your kinswoman— she also has conceived a son in her old age; and this month is the sixth to her who was called barren.” Do you see that the barren one was for the sake of the Virgin? Since with what object did he adduce to her the child-bearing of her kinswoman? With what object did he say, “in her old age?” with what object did he add, “who was called barren?” It was by way of inducing her by all these things, manifestly, to the believing the glad annunciation.
For this cause he spoke of both the age and the disabling effect of nature; for this cause he awaited the time also which had elapsed from the conception; for he did not tell to her the glad tidings immediately from the beginning, but awaited for a six-months period to have passed to the barren one, in order that the puerperal swelling might, for the rest, be a pledge of the pregnancy, and an indisputable demonstration might arise of the conception. And pray again look at the intelligence of Gabriel.
For he neither reminded her of Sarah, nor of Rebecca, nor of Rachel; and yet they also were barren, and they had grown old, and that which took place was a marvel; but the stories were ancient. Now things new and recent and occurring in our generation are wont to induce us into the belief of marvels more than those which are old. On this account having let those women alone, that she should understand from her kinswoman Elizabeth herself what was coming upon her, he brought it forward; so as from her to lead her to her own— that most awful and august childbirth.
For the child-birth of the barren one lay between ours and that of the Master less indeed than that of the Virgin, but greater than ours. On this account it was by Elizabeth lying between, just as by some bridge, that he lifted up the mind of the Virgin from the travail which is according to nature, to that which is above nature.
Source: Homily Against Publishing the Errors of the Brethren (New Advent)