Secondly, that thus she might be safeguarded from ill fame. Whence Ambrose says on Luke 1:26, 27: "She was espoused lest she be wounded by the ill-fame of violated virginity, in whom the pregnant womb would betoken corruption."
Thirdly, that, as Jerome says, Joseph might administer to her wants.
This was fitting, again, for our sake. First, because Joseph is thus a witness to Christ's being born of a virgin. Wherefore Ambrose says: "Her husband is the more trustworthy witness of her purity, in that he would deplore the dishonor, and avenge the disgrace, were it not that he acknowledged the mystery."
Secondly, because thereby the very words of the Virgin are rendered more credible by which she asserted her virginity. Thus Ambrose says: "Belief in Mary's words is strengthened, the motive for a lie is removed. If she had not been espoused when pregnant, she would seem to have wished to hide her sin by a lie: being espoused, she had no motive for lying, since a woman's pregnancy is the reward of marriage and gives grace to the nuptial bond." These two reasons add strength to our faith.
Thirdly, that all excuse be removed from those virgins who, through want of caution, fall into dishonor. Hence Ambrose says: "It was not becoming that virgins should expose themselves to evil report, and cover themselves with the excuse that the Mother of the Lord had also been oppressed by ill-fame."
Fourthly, because by this the universal Church is typified, which is a virgin and yet is espoused to one Man, Christ, as Augustine says (De Sanct. Virg. xii).
A fifth reason may be added: since the Mother of the Lord being both espoused and a virgin, both virginity and wedlock are honored in her person, in contradiction to those heretics who disparaged one or the other.
Reply Obj. 1: We must believe that the Blessed Virgin, Mother of God, desired, from an intimate inspiration of the Holy Ghost, to be espoused, being confident that by the help of God she would never come to have carnal intercourse: yet she left this to God's discretion. Wherefore she suffered nothing in detriment to her virginity.
Reply Obj. 2: As Ambrose says on Luke 1:26: "Our Lord preferred that men should doubt of His origin rather than of His Mother's purity. For he knew the delicacy of virgin modesty, and how easily the fair name of chastity is disparaged: nor did He choose that our faith in His Birth should be strengthened in detriment to His Mother." We must observe, however, that some miracles wrought by God are the direct object of faith; such are the miracles of the virginal Birth, the Resurrection of our Lord, and the Sacrament of the Altar. Wherefore our Lord wished these to be more hidden, that belief in them might have greater merit. Whereas other miracles are for the strengthening of faith: and these it behooves to be manifest.
Reply Obj. 3: As Augustine says (De Trin. iii), the devil can do many things by his natural power which he is hindered by the Divine power from doing. Thus it may be that by his natural power the devil could know that the Mother of God knew not man, but was a virgin; yet was prevented by God from knowing the manner of the Divine Birth. That afterwards the devil after a fashion knew that He was the Son of God, makes no difficulty: because then the time had already come for Christ to make known His power against the devil, and to suffer persecution aroused by him. But during His infancy it behooved the malice of the devil to be withheld, lest he should persecute Him too severely: for Christ did not wish to suffer such things then, nor to make His power known, but to show Himself to be in all things like other infants. Hence Pope Leo (Serm. in Epiph. iv) says that "the Magi found the Child Jesus small in body, dependent on others, unable to speak, and in no way differing from the generality of human infants." Ambrose, however, expounding Luke 1:26, seems to understand this of the devil's members. For, after giving the above reason—namely, that the prince of the world might be deceived—he continues thus: "Yet still more did He deceive the princes of the world, since the evil disposition of the demons easily discovers even hidden things: but those who spend their lives in worldly vanities can have no acquaintance of Divine things."