Objection 1: It would seem that Christ should not have been born of an espoused virgin. For espousals are ordered to carnal intercourse. But our Lord's Mother never wished to have carnal intercourse with her husband; because this would be derogatory to the virginity of her mind. Therefore she should not have been espoused.
Obj. 2: Further, that Christ was born of a virgin was miraculous, whence Augustine says (Ep. ad Volus. cxxxvii): "This same power of God brought forth the infant's limbs out of the virginal womb of His inviolate Mother, by which in the vigor of manhood He passed through the closed doors. If we are told why this happened, it will cease to be wonderful; if another instance be alleged, it will no longer be unique." But miracles that are wrought in confirmation of the Faith should be manifest. Since, therefore, by her Espousals this miracle would be less evident, it seems that it was unfitting that Christ should be born of an espoused virgin.
Obj. 3: Further, the martyr Ignatius, as Jerome says on Matt. 1:18, gives as a reason of the espousals of the Mother of God, "that the manner of His Birth might be hidden from the devil, who would think Him to be begotten not of a virgin but of a wife." But this seems to be no reason at all. First, because by his natural cunning he knows whatever takes place in bodies. Secondly, because later on the demons, through many evident signs, knew Christ after a fashion: whence it is written (Mk. 1:23, 24): "A man with an unclean spirit . . . cried out, saying: What have we to do with Thee, Jesus of Nazareth? Art Thou come to destroy us? I know . . . Thou art the Holy one of God." Therefore it does not seem fitting that the Mother of God should have been espoused.
Obj. 4: Further, Jerome gives as another reason, "lest the Mother of God should be stoned by the Jews as an adulteress." But this reason seems to have no weight, for if she were not espoused, she could not be condemned for adultery. Therefore it does not seem reasonable that Christ should be born of an espoused virgin.
On the contrary, It is written (Matt. 1:18): "When as His Mother Mary was espoused to Joseph": and (Luke 1:26, 27): "The angel Gabriel was sent . . . to a virgin espoused to a man whose name was Joseph."
I answer that, It was fitting that Christ should be born of an espoused virgin; first, for His own sake; secondly, for His Mother's sake; thirdly, for our sake. For the sake of Christ Himself, for four reasons. First, lest He should be rejected by unbelievers as illegitimate: wherefore Ambrose says on Luke 1:26, 27: "How could we blame Herod or the Jews if they seem to persecute one who was born of adultery?"
Secondly, in order that in the customary way His genealogy might be traced through the male line. Thus Ambrose says on Luke 3:23: "He Who came into the world, according to the custom of the world had to be enrolled. Now for this purpose, it is the men that are required, because they represent the family in the senate and other courts. The custom of the Scriptures, too, shows that the ancestry of the men is always traced out."
Thirdly, for the safety of the new-born Child: lest the devil should plot serious hurt against Him. Hence Ignatius says that she was espoused "that the manner of His Birth might be hidden from the devil."
Fourthly, that He might be fostered by Joseph: who is therefore called His "father," as bread-winner.
It was also fitting for the sake of the Virgin. First, because thus she was rendered exempt from punishment; that is, "lest she should be stoned by the Jews as an adulteress," as Jerome says.