January 1st, Solemnity of Mary the Holy Mother of God
The generation of Christ was in this wise. When his mother Mary was espoused to Joseph etc. (Matthew 1:18)
St. Matthew makes it very clear that Mary is truly the Mother of Jesus, and this is affirmed also in the other Gospels many times over. Throughout the Gospels and in the Church’s Tradition, Mary is called the Mother of Jesus. Indeed, we know that (because Jesus is one divine person) Mary is truly said to be the Mother of God.
However, given that Mary is the Mother of Jesus with respect to his humanity, why do we not call the Holy Spirit the Father of Jesus? Since it was by the power of the Holy Spirit that Mary conceived, and since Mary is called the spouse of the Holy Spirit, why does the Church refuse to say that Jesus is the Son of the Holy Spirit in his humanity?
Mary is truly the Mother of Jesus
A handful of times in the Gospels, St. Joseph is called the “father” of Jesus – by St. Luke (2:33), and even by Mary herself (Thy father and I have sought thee sorrowing, Luke 2:48) – but it is quite clear that the term “father” is not used literally in these cases. St. Luke himself specifies that St. Joseph was only thought to be the father of Jesus, since the Holy Family kept the mystery of the Incarnation hidden from the public and most thought that the marriage between Joseph and Mary was a natural one – [Jesus] being (as it was supposed) the son of Joseph (Luke 3:23).
However, when we come to the Blessed Virgin Mary, it is quite clear that the Gospels and the rest of the New Testament (as well as the prophecies of the Old Testament) present her as the true and natural Mother of our Savior. The Lord Jesus was truly born of the Virgin Mary, and this is affirmed also by St. Paul – God sent his Son, made of a woman (Galatians 4:4).
Christ’s body was not brought down from heaven, nor was it formed of the earth, but rather (by the power of the Holy Spirit) he took flesh of the Blessed Virgin Mary. Indeed, we ought to suppose that (on the part of the Woman) all was natural – St. Thomas Aquinas held that Christ was formed of the blood of Mary, but by that he meant that the Savior came from the normal reproductive material of his Mother (for, in those days, it was thought that the blood of the woman filled something of the role we assign to the egg). What is most important to hold is that Christ took his flesh from the body of his Mother Mary; accepting this, any explanation will suffice.
The angelic St. Thomas specifies that, in respect to what belongs to the woman, Christ’s conception was in accord with the laws of nature – hence the Blessed Mary supplied the matter just as any mother would. Now, in regards the working of the Holy Spirit, Christ’s conception was above the laws of nature – hence, he was conceived and born of the Virgin. [cf. ST III, q.31, a.5, here]
Now, all that is required for motherhood is that the woman supply the matter (i.e. the egg), and this Mary did. Further, she nurtured him through the process of gestation and gave birth to him – and, although this birth was miraculous (since it did not harm her virginal integrity, but rather Christ passed through her as light through glass), it was nevertheless a true and real birth. Therefore, the Blessed Virgin is truly Christ’s mother in relation to his humanity. [cf. ST III, q.35, a.3, here]
Further, though the birth of our Savior was miraculous, this can in no way be said to reduce the motherhood of Blessed Mary – indeed, a woman who brings forth a child through c-section is still called a mother; hence, the violent passage of the infant body through the birth canal is not necessary for motherhood. In any case, the virginal birth of our Lord is still a true birth, though it exceeds the laws of nature and is a most wondrous and unique miracle.
All that being said, Mary is the “Mother of Jesus” not simply because she gave him birth, but most especially because she conceived him in her most pure womb, nourished him with her body, and bore him for nine months in her virginal cloister.
Still, we must be very careful to point out that the Savior existed before his Mother, for he did not begin to exist when he was conceived, but rather assumed to himself a human nature in that moment – remaining what he was (God), he became what he was not (Man).
What “father” and “mother” mean
Before continuing, it will be helpful to briefly state what we mean when we employ the words “father” and “mother” as well as “son” and “daughter”.
These words, says the Angel of the Schools (St. Thomas) “result from generation; yet not from any generation, but from that of living things, especially animals. For we do not say that fire generated is the son of the fire generating it, except, perhaps, metaphorically; we speak thus only of animals in whom generation is more perfect.
“Nevertheless, the word ‘son’ is not applied to everything generated in animals, but only to that which is generated into likeness of the generator. Wherefore, as Augustine says (Enchiridion xxxix), we do not say that a hair which is generated in a man is his son; nor do we say that a man who is born is the son of the seed; for neither is the hair like the man nor is the man born like the seed, but like the man who begot him.” [ST III, q.32, a.3, here]
Jesus is the “Son of God”
Now, any man can be called the son of God in two respects: First, on account of the fact that man is created in the image of God; second, on account of the regeneration of the soul through grace by which the likeness to God is restored in man. And in this respect also, the angels are (in Sacred Scripture) occasionally called “sons of God” – for they bear the image of God in an even more excellent manner, and they have greater grace as being among the blessed than do we who are still on earth.
However, a man is called a “son [or child] of God” in an imperfect sense, for he is not a perfect image of God, but is a mere creature. Yet, a man is called the son of his natural human father in a perfect sense, for he is truly of the same nature as his human father.
But Christ alone is called the “Son of God” in the perfect sense, for he is equal and co-eternal with the Father, the two being of one nature. Hence, in respect of his divinity, Jesus is rightly called the Son of God, meaning the Son of God the Father – for the one Person (the Word) was begotten of the Father from all eternity.
The Holy Spirit is the author of Christ’s conception, but is not his Father
St. Augustine says (Enchiridion xl): “Christ was born of the Holy Ghost not as a Son, and of the Virgin Mary as a Son.”
Now, Christ was conceived by the power of the Holy Spirit, but we do not say that the Holy Spirit supplied male seed (such would be an impious blasphemy). Rather, by divine power, the Blessed Trinity took matter (i.e. the egg) from the Blessed Virgin and made it to become man (forming the matter by the infusion of the rational human soul), assumed at that very moment by the person of the Word.
But, in his humanity, Christ is not of the same nature as the Holy Spirit – for neither his body nor his soul is consubstantial with the Holy Spirit. Therefore, the Holy Spirit cannot possibly be said to be the “father” of Jesus in the way that a man is the father of his son.
Further, since Christ is already the perfect Son of God the Father, we do not predicate of him a secondary sonship according to grace – while we are “sons” by adoption through grace, Jesus is “Son” by nature; since he is only one person (the Word and Son of the Father). Thus, not even according to grace, do we call Jesus the son of the Holy Spirit – since, he is already perfectly the Son of God.
Put simply, by St. Thomas
The Common Doctor states all this in the simplest terms [ST III, q.32, a.3, ad1, here]
“Christ was conceived of the Virgin Mary, who supplied the matter of His conception unto likeness of species. For this reason He is called her Son. But as man He was conceived of the Holy Ghost as the active principle of His conception, but not unto likeness of species, as a man is born of his father. Therefore Christ is not called the Son of the Holy Ghost.”