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The issue of the virginity of Mary, the mother of Jesus, has been debated endlessly for about 2,000 years and continues to be debated endlessly today. I've read many arguments, both for the position that Mary could not possibly have been a virgin and for the position that she must have been.
Nonetheless, I'm interested to read what the Fathers of the first-millennium Church thought about these things. In the Catena Aurea, some of their thoughts are compiled as a commentary on the following verse from the Gospel of Matthew:
Now the birth of Jesus Christ was on this wise: When as His mother Mary was espoused to Joseph, before they came together, she was found with child of the Holy Ghost.
One of the things noted by more than one of the Fathers is that this way of describing Christ's conception is quite different from how the preceding genealogies described the conception of his ancestors. For example, St. John Chrysostom and Remigius, a priest and monk of Auxerre who wrote Biblical commentaries based on the work of earlier Christian scholars:
Chrys. He announces that he is to relate the manner of generation, shewing therein that he is about to speak of some new thing; that you may not suppose when you hear mention of Mary's husband, that Christ was born by the law of nature. Remig. Yet it might be referred to the foregoing in this way, The generation of Christ was, as I have related, thus, Abraham begat Isaac.
Here, Remigius and St. John Chrysostom observe that the Gospel writer does not simply use the default expression for the conception of Jesus: "X begat Y." Instead of writing in the same well-known way he had just written many, many times for the genealogy of Christ, he writes of a great discovery. Namely, that Jesus was conceived by the power of the Holy Spirit.
So why, if Jesus was the Son of God, conceived of the Holy Spirit, did he need an earthly father? Doesn't that detract from his uniqueness and suggest that Jesus was conceived by more standard sexual means?
St. Jerome and others were answering this question back in the 300s and earlier, well before Islamic thinkers who noticed that the Qur'an doesn't say anything about Joseph in the infancy narrative of Jesus might have pointed out the discrepancy between the texts of the Gospel and the Qur'an and been prompted to inquire as to why Joseph would be needed in God's plan of salvation.
Jerome; But why is He conceived not of a Virgin merely, but of a Virgin espoused? First, that by the descent of Joseph, Mary's family might be made known; secondly, that she might not be stoned by the Jews as an adulteress; thirdly, that in her flight into Egypt that she might have the comfort of a husband. The Martyr Ignatius adds yet a fourth reason, namely, that his birth might be hid from the Devil, looking for him to be born of a wife and not of a virgin.
St. Peter Chrysologus adds that:
If you are not confounded when you hear of the birth of God, let not his conception disturb you, seeing the pure virginity of the mother removes all that might shock human reverence. And what offence against our awe and reverence is there, when the Deity entered into union with purity that was always dear to Him, where an Angel is mediator, faith is bridesmaid, where chastity is the giving away, virtue the gift, conscience the judge, God the cause; where the conception is inviolateness, the birth virginity, and the mother a virgin.
Chrysologus also points us to another concern, which is why the virginity of Mary is important in the grand scheme of salvation history in which God's only-begotten Son enters into the world.
Origen of Alexandria and St. Augustine of Hippo can help us with this concern. Origen makes a distinction between being espoused (or bethrothed) and united in wedlock via sexual consummation of their union, which generally did not happen immediately upon betrothal.
Origen; She was indeed espoused to Joseph, but not united in wedlock; that is to say, His mother immaculate, His mother incorrupt, His mother pure. His mother! Whose mother? The mother of God, of the Only-Begotten, of the Lord, of the King, of the Maker of all things, and the Redeemer of all.
Aug. There was no carnal knowledge in this wedlock, because in sinful flesh this could not be without carnal desire which came of sin, and which He would be without, who was to be without sin; and that hence He might teach us that all flesh which is born of sexual union is sinful flesh, seeing that Flesh alone was without sin, which was not so born.
St. Augustine points out that in order for Jesus Christ to be the sinless Son of God, the spotless Lamb who was sacrificed for us, he would need to enter the world in such a way as to avoid inheriting the stain of original sin. And that in order for Christ's sinlessness to be made clear, he would need to be conceived in a different way than the normal way which was known to pass on the sinfulness inherited from the Fall.
But does that also make Mary someone special for having born Him in her womb and raised Him, or was she a mere fleshly vessel? St. Cyril of Alexandria helps us to think it through with the mind of the Early Church.
Cyril; What will any one see in the Blessed Virgin more than in other mothers, if she be not the mother of God, but of Christ, or the Lord, as Nestorius says? For it would not be absurd should any one please to name the mother of any anointed person, the mother of Christ. Yet she alone and more than they is called the Holy Virgin, and the mother of Christ. For she bare not a simple man as ye say, but rather the Word incarnate, and made man of God the Father. But perhaps you say, Tell me, do you think the Virgin was made the mother of His divinity? To this also we say, that the Word was born of the very substance of God Himself, and without beginning of time always coexisted with the Father. But in these last times when He was made flesh, that is united to flesh, having a rational soul, He is said to be born of a woman after the flesh. Yet is this sacrament in a manner brought out like to birth among us; for the mothers of earthly children impart to their nature that flesh that is to be perfected by degrees in the human form; but God sends the life into the animal. But though these are mothers only of the earthly bodies, yet when they bear children, they are said to bear the whole animal, and not a part of it only. Such do we see to have been done in the birth of Emmanuel; the Word of God was born of the substance of His Father; but because He took on Him flesh, making it His own, it is necessary to confess that he was born of a woman according to the flesh. Where seeing He is truly God, how shall anyone doubt to call the Holy Virgin the Mother of God?
Cyril was writing in response to the heresies of Nestorius, who denied that Mary was the bearer of Christ in both His divine and human natures. Nestorius seemed inclined to take the view that Mary was bearer only of Christ's human nature.
Cyril argues that this is preposterous, because it doesn't match with how we understand the motherhood of anyone else. Though God creates the soul of a person directly, we don't say that a person's mother is only mother of that person's body. Indeed, the mother bears and bears with the child in its totality, body and soul as a united whole.
In the same way, the mother of Christ bore her son Jesus in His totality, humanity and divinity united in holiness. Thus, Cyril concludes (as do many other early Church Fathers at the Council of Ephesus and prior) that Mary is indeed the Mother of God (Theotokos is Cyril's preferred term).
And thus was formalized what many Christians had understood intuitively before: that Mary is the Mother of God. Though Jesus, the only-begotten Son of God, was undoubtedly conceived of the Holy Spirit, at least according to the Early Church, the only-begotten Son of God was also undoubtedly born of a woman named Mary according to the Early Church.
Note: The above is a picture of my copy of the Catena Aurea (Volume 1: St. Matthew).