4 But if, when their mouths are stopped on this point, they should seek another, namely, what is said touching Mary's virginity, and should object to us other translators, saying, that they used not the term “virgin,” but “young woman;” in the first place we will say this, that the Seventy were justly entitled to confidence above all the others. For these made their translation after Christ's coming, continuing to be Jews, and may justly be suspected as having spoken rather in enmity, and as darkening the prophecies on purpose; but the Seventy, as having entered upon this work an hundred years or more before the coming of Christ, stand clear from all such suspicion, and on account of the date, and of their number, and of their agreement, would have a better right to be trusted.
But even if they bring in the testimony of those others, yet so the tokens of victory would be with us. Because the Scripture is wont to put the word “youth,” for “virginity;” and this with respect not to women only, but also to men. For it is said, “young men and maidens, old men with younger ones.” And again, speaking of the damsel who is attacked, it says, “if the young woman cry out,” meaning the virgin.
And what goes before also establishes this interpretation. For he does not merely say, “Behold, the Virgin shall be with child,” but having first said, “Behold, the Lord Himself shall give you a sign,” then he subjoins, “Behold, the Virgin shall be with child.” Whereas, if she that was to give birth was not a virgin, but this happened in the way of marriage, what sort of sign would the event be? For that which is a sign must of course be beyond the course of common events, it must be strange and extraordinary; else how could it be a sign?
5. “Then Joseph, being raised from sleep, did as the angel of the Lord had bidden him.” Do you see obedience, and a submissive mind? Do you see a soul truly wakened, and in all things incorruptible? For neither when he suspected something painful or amiss could he endure to keep the Virgin with him; nor yet, after he was freed from this suspicion, could he bear to cast her out, but he rather keeps her with him, and ministers to the whole Dispensation.
“And took unto him Mary his wife.” Do you see how continually the evangelist uses this word, not willing that that mystery should be disclosed as yet, and annihilating that evil suspicion?
And when he had taken her, “he knew her not, till she had brought forth her first-born Son.” He has here used the word “till,” not that you should suspect that afterwards he did know her, but to inform you that before the birth the Virgin was wholly untouched by man. But why then, it may be said, has he used the word, “till”? Because it is usual in Scripture often to do this, and to use this expression without reference to limited times. For so with respect to the ark likewise, it is said, “The raven returned not till the earth was dried up.” And yet it did not return even after that time. And when discoursing also of God, the Scripture says, “From age until age You are,” not as fixing limits in this case. And again when it is preaching the Gospel beforehand, and saying, “In his days shall righteousness flourish, and abundance of peace, till the moon be taken away,” it does not set a limit to this fair part of creation. So then here likewise, it uses the word “till,” to make certain what was before the birth, but as to what follows, it leaves you to make the inference. Thus, what it was necessary for you to learn of Him, this He Himself has said; that the Virgin was untouched by man until the birth; but that which both was seen to be a consequence of the former statement, and was acknowledged, this in its turn he leaves for you to perceive; namely, that not even after this, she having so become a mother, and having been counted worthy of a new sort of travail, and a child-bearing so strange, could that righteous man ever have endured to know her. For if he had known her, and had kept her in the place of a wife, how is it that our Lord commits her, as unprotected, and having no one, to His disciple, and commands him to take her to his own home?
How then, one may say, are James and the others called His brethren? In the same kind of way as Joseph himself was supposed to be husband of Mary. For many were the veils provided, that the birth, being such as it was, might be for a time screened. Wherefore even John so called them, saying, “For neither did His brethren believe in Him.”
Source: Homilies on the Gospel of St. Matthew (New Advent)