12 Then having obscurely signified the matter in hand, he mentioned not the evil suspicion; but, in a manner more reverent and seemly, by telling the cause of travail he removed this also; implying that the very thing which had made him afraid, and for which he would have cast her out—this very thing, I say, was a just cause why he should take her and retain her in his house. Thus more than entirely doing away with his distress. “For she is not only free,” says he, “from unlawful intercourse, but even above all nature is her conception. Not only therefore put away your fear, but even rejoice more exceedingly, 'for that which is conceived in her is of the Holy Ghost.'”
A strange thing it was which he spoke of, surpassing man's reason, and above all the laws of nature. How then is he to believe, to whom such tidings are altogether new? “By the things that are past,” says he, “by the revelations.” For with this intent he laid open all things that were in his mind, what he felt, what he feared, what he was resolved to do—that by these he might assure himself of this point.
Or rather, not by things past only, but like wise by things to come, he wins him over. “And she shall bring forth,” says he, “a Son, and you shall call His name Jesus.” For do not thou, because He is of the Holy Ghost, imagine that you are an alien to the ministry of this dispensation. Since although in the birth you have no part, but the Virgin abode untouched, nevertheless, what pertains to a father, not injuring the honor of virginity, that do I give you, to set a Name on that which is born: for “you shall call Him.” For though the offspring be not yours, yet shall you exhibit a father's care towards Him. Wherefore I do straightway, even from the giving of the name, connect you with Him that is born.
Then lest on the other hand any one should from this suspect him to be the father, hear what follows, with what exact care he states it. “She shall bring forth,” he says, “a Son:” he does not say, “bring forth to you,” but merely “she shall bring forth,” putting it indefinitely: since not to him did she bring forth, but to the whole world.
Source: Homilies on the Gospel of St. Matthew (New Advent)