Objection 1. It would seem that Christ's Mother did not remain a virgin after His Birth. For it is written (Matthew 1:18): "Before Joseph and Mary came together, she was found with child of the Holy Ghost." Now the Evangelist would not have said this--"before they came together"--unless he were certain of their subsequent coming together; for no one says of one who does not eventually dine "before he dines" (cf. Jerome, Contra Helvid.). It seems, therefore, that the Blessed Virgin subsequently had intercourse with Joseph; and consequently that she did not remain a virgin after (Christ's) Birth.
Objection 2. Further, in the same passage (Matthew 1:20) are related the words of the angel to Joseph: "Fear not to take unto thee Mary thy wife." But marriage is consummated by carnal intercourse. Therefore it seems that this must have at some time taken place between Mary and Joseph: and that, consequently she did not remain a virgin after (Christ's) Birth.
Objection 3. Further, again in the same passage a little further on (Matthew 1:24-25) we read: "And" (Joseph) "took unto him his wife; and he knew her not till she brought forth her first-born Son." Now this conjunction "till" is wont to designate a fixed time, on the completion of which that takes place which previously had not taken place. And the verb "knew" refers here to knowledge by intercourse (cf. Jerome, Contra Helvid.); just as (Genesis 4:1) it is said that "Adam knew his wife." Therefore it seems that after (Christ's) Birth, the Blessed Virgin was known by Joseph; and, consequently, that she did not remain a virgin after the Birth (of Christ).
Objection 4. Further, "first-born" can only be said of one who has brothers afterwards: wherefore (Romans 8:29): "Whom He foreknew, He also predestinated to be made conformable to the image of His Son; that He might be the first-born among many brethren." But the evangelist calls Christ the first-born by His Mother. Therefore she had other children after Christ. And therefore it seems that Christ's Mother did not remain a virgin after His Birth.
Objection 5. Further, it is written (John 2:12): "After this He went down to Capharnaum, He"--that is, Christ--"and His Mother and His brethren." But brethren are those who are begotten of the same parent. Therefore it seems that the Blessed Virgin had other sons after Christ.
Objection 6. Further, it is written (Matthew 27:55-56): "There were there"--that is, by the cross of Christ--"many women afar off, who had followed Jesus from Galilee, ministering unto Him; among whom was Mary Magdalen, and Mary the mother of James and Joseph, and the mother of the sons of Zebedee." Now this Mary who is called "the mother of James and Joseph" seems to have been also the Mother of Christ; for it is written (John 19:25) that "there stood by the cross of Jesus, Mary His Mother." Therefore it seems that Christ's Mother did not remain a virgin after His Birth.
On the contrary, It is written (Ezekiel 44:2): "This gate shall be shut, it shall not be opened, and no man shall pass through it; because the Lord the God of Israel hath entered in by it." Expounding these words, Augustine says in a sermon (De Annunt. Dom. iii): "What means this closed gate in the House of the Lord, except that Mary is to be ever inviolate? What does it mean that 'no man shall pass through it,' save that Joseph shall not know her? And what is this--'The Lord alone enters in and goeth out by it'--except that the Holy Ghost shall impregnate her, and that the Lord of angels shall be born of her? And what means this--'it shall be shut for evermore'--but that Mary is a virgin before His Birth, a virgin in His Birth, and a virgin after His Birth?"
I answer that, Without any hesitation we must abhor the error of Helvidius, who dared to assert that Christ's Mother, after His Birth, was carnally known by Joseph, and bore other children. For, in the first place, this is derogatory to Christ's perfection: for as He is in His Godhead the Only-Begotten of the Father, being thus His Son in every respect perfect, so it was becoming that He should be the Only-begotten son of His Mother, as being her perfect offspring.
Secondly, this error is an insult to the Holy Ghost, whose "shrine" was the virginal womb ["Sacrarium Spiritus Sancti" (Office of B. M. V., Ant. ad Benedictus, T. P.), wherein He had formed the flesh of Christ: wherefore it was unbecoming that it should be desecrated by intercourse with man.
Thirdly, this is derogatory to the dignity and holiness of God's Mother: for thus she would seem to be most ungrateful, were she not content with such a Son; and were she, of her own accord, by carnal intercourse to forfeit that virginity which had been miraculously preserved in her.
Fourthly, it would be tantamount to an imputation of extreme presumption in Joseph, to assume that he attempted to violate her whom by the angel's revelation he knew to have conceived by the Holy Ghost.
We must therefore simply assert that the Mother of God, as she was a virgin in conceiving Him and a virgin in giving Him birth, did she remain a virgin ever afterwards.
Reply to Objection 1. As Jerome says (Contra Helvid. i): "Although this particle 'before' often indicates a subsequent event, yet we must observe that it not infrequently points merely to some thing previously in the mind: nor is there need that what was in the mind take place eventually, since something may occur to prevent its happening. Thus if a man say: 'Before I dined in the port, I set sail,' we do not understand him to have dined in port after he set sail: but that his mind was set on dining in port." In like manner the evangelist says: "Before they came together" Mary "was found with child, of the Holy Ghost," not that they came together afterwards: but that, when it seemed that they would come together, this was forestalled through her conceiving by the Holy Ghost, the result being that afterwards they did not come together.
Reply to Objection 2. As Augustine says (De Nup. et Concup. i): "The Mother of God is called (Joseph's) wife from the first promise of her espousals, whom he had not known nor ever was to know by carnal intercourse." For, as Ambrose says on Luke 1:27: "The fact of her marriage is declared, not to insinuate the loss of virginity, but to witness to the reality of the union."
Reply to Objection 3. Some have said that this is not to be understood of carnal knowledge, but of acquaintance. Thus Chrysostom says [Opus Imperf. in Matth., Hom. 1: among the spurious works ascribed to Chrysostom] that "Joseph did not know her, until she gave birth, being unaware of her dignity: but after she had given birth, then did he know her. Because by reason of her child she surpassed the whole world in beauty and dignity: since she alone in the narrow abode of her womb received Him Whom the world cannot contain."
Others again refer this to knowledge by sight. For as, while Moses was speaking with God, his face was so bright "that the children of Israel could not steadfastly behold it"; so Mary, while being "overshadowed" by the brightness of the "power of the Most High," could not be gazed on by Joseph, until she gave birth. But afterwards she is acknowledged by Joseph, by looking on her face, not by lustful contact.
Jerome, however, grants that this is to be understood of knowledge by intercourse; but he observes that "before" or "until" has a twofold sense in Scripture. For sometimes it indicates a fixed time, as Galatians 3:19: The law "was set because of transgressions, until the seed should come, to whom He made the promise." On the other hand, it sometimes indicates an indefinite time, as in Psalm 122:2: "Our eyes are unto the Lord our God, until He have mercy on us"; from which it is not to be gathered that our eyes are turned from God as soon as His mercy has been obtained. In this sense those things are indicated "of which we might doubt if they had not been written down: while others are left out to be supplied by our understanding. Thus the evangelist says that the Mother of God was not known by her husband until she gave birth, that we may be given to understand that still less did he know her afterwards" (Adversus Helvid. v).
Reply to Objection 4. The Scriptures are wont to designate as the first-born, not only a child who is followed by others, but also the one that is born first. "Otherwise, if a child were not first-born unless followed by others, the first-fruits would not be due as long as there was no further produce" [Jerome, Adversus Helvid. x]: which is clearly false, since according to the law the first-fruits had to be redeemed within a month (Numbers 18:16).
Reply to Objection 5. Some, as Jerome says on Matthew 12:49-50, "suppose that the brethren of the Lord were Joseph's sons by another wife. But we understand the brethren of the Lord to be not sons of Joseph, but cousins of the Saviour, the sons of Mary, His Mother's sister." For "Scripture speaks of brethren in four senses; namely, those who are united by being of the same parents, of the same nation, of the same family, by common affection." Wherefore the brethren of the Lord are so called, not by birth, as being born of the same mother; but by relationship, as being blood-relations of His. But Joseph, as Jerome says (Contra Helvid. ix), is rather to be believed to have remained a virgin, "since he is not said to have had another wife," and "a holy man does not live otherwise than chastely."
Reply to Objection 6. Mary who is called "the mother of James and Joseph" is not to be taken for the Mother of our Lord, who is not wont to be named in the Gospels save under this designation of her dignity--"the Mother of Jesus." This Mary is to be taken for the wife of Alphaeus, whose son was James the less, known as the "brother of the Lord" (Galatians 1:19).
Source: Summa Theologica (New Advent)