By St. Ambrose of Milan
(Commentary on Luke, Bk. 2)
The mysteries of God are unsearchable, as is especially declared in the prophetic words: "What man is he that can know the counsel of God? or who can think what the will of the Lord is?"
Nevertheless, some things have been revealed to us. And hence we may gather, from the words and works of our Lord and Savior, that there was a special purpose of God in the fact that she who was chosen to bring forth the Lord was espoused to a man.
Why did not the power of the Highest overshadow her before she was so espoused? Perhaps it was lest any might blasphemously say that the Holy One was conceived in fornication.
"And the Angel came in unto her." Let us learn from this Virgin how to bear ourselves; let us learn by her devout utterance; above all let us learn by the holy mystery to be timid, to avoid the advances of men, and to shrink from men's addresses. Would that our women would learn from the example of modesty here set before us. She upon whom the stare of men had never been fixed was alone in her chamber, and was found only by an Angel. There was neither companion nor witness there, that what passed might not be debased in gossip; and the Angel saluted her.
The message of God to the Virgin was a mystery so great that it must needs not be uttered by the mouth of man, but only by an Angel. For the first time on earth the words are spoken: "The Holy Spirit shall come upon thee." The holy maiden heard, and believed.
At length she said: "Behold the handmaiden of the Lord; be it unto me according to thy word." Here is an example of lowliness, here is a pattern of true devotion. At the very moment she is chosen to be the Mother of the Lord she declares herself to be his handmaid. The knowledge that she was chosen to this high vocation wrought in Mary only an act of humility.
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St. Jerome
(Commentary on Matthew, Bk. 1)
Why was the Lord conceived of a virgin already espoused, rather than of one as yet unpledged to a man?
For one thing, because from the genealogy of Joseph the lineage of Mary as a descendant of David, and thus of her Child, could be the more easily established.
For another, because by this betrothal Mary would be saved from being stoned by the Jews as an adulteress.
Again, because thereby Mary was given a guardian during the flight into Egypt.
To these reasons the Martyr Ignatius adds another, namely; that the virgin birth might take place unknown to the devil, who would naturally suppose that Mary had conceived by Joseph.
"Before they came together, she was found with child, of the Holy Spirit." That is, she was found so to be by Joseph, not by any one else, for he already had almost an husband's privilege to know all that concerned her. But when it is said: "Before they came together," it does not follow that they ever did come together carnally. The Scripture is to be understood merely in the sense that up to this time they had not done so.
"Then Joseph her husband, being a just man, and not willing to make her a public example, was minded to put her away privately." If any man be joined to a woman with whom he has lived in fornication, they in such wise become one person, that, according to the Law, they both are guilty of the crime if they both be privy to it. How then can it be that Joseph is described as a just man, at the very time he was compounding criminality in his espoused wife, to save her from the punishment demanded by the Law? This is a testimonial to Mary. For it indicates that he knew her to be a pure virgin, and that even though he understood not the mystery of her pregnancy, yet on account of his entire trust in her, he was willing to hold his peace.
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St. Leo the Great
(Sermon 2 on the Nativity of the Lord)
God is He whose nature is goodness, whose will is power, and whose work is mercy.
Wherefore, at the very beginning of the world, as soon as the devil's hatred had mortally poisoned mankind with the venom on his envy, this almighty and merciful God even then foretold those remedies which his mercy had foreordained for our healing.
At that time he bade the serpent know that there was to be a seed of the woman who yet should crush the prideful swelling of his pestilential head. This seed was none other than the Christ to come in the flesh, even God and Man in one Person, who should be born of the Virgin, and by his virgin birth should condemn the seducer of man.
The devil rejoiced that he had, by his artful cunning, so deceived man as to make him lose the gifts of God, and forfeit the privilege of eternal life. Yea, when the devil had thus brought man under the hard sentence of death, he found a certain solace for his own misery in the fact that he now had a comrade in his guilt. He thought also that God, in His just anger, would change His original design towards man, whom he had made in such honor.
But, dearly beloved, that unchangeable God, whose will cannot be balked of its loving-kindness, in the dispensation of his own secret counsel, had already provided a mysterious way for carrying out his original purpose of goodness. So it was that mankind, which had been led into sin by the wicked craft of the devil, was not suffered to perish, and frustrate that gracious purpose of God.