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 Obj. 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 Obj. 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 Obj. 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 Gal. 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 Ps. 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).