Thomas Aquinas, Compendium of Theology

If Mary was thus strengthened against every movement of sin by her first sanctification, much more did grace grow in her and much more was concupiscence weakened or even completely uprooted in her, when the Holy Ghost came upon her, according to the angel's word, to form of her the body of Christ. After she had been made the shrine of the Holy Spirit and the tabernacle of the Son of God, we may not believe that there was ever any inclination to sin in her, or that she ever experienced any pleasurable feeling of carnal concupiscence. And so we must view with revulsion the error of Helvidius who, while admitting that Christ was conceived and born of the Virgin, asserted that she later bore other sons to Joseph.

Certainly this error finds no support in Matthew's statement that Joseph "knew her not" namely, Mary, "till she brought forth her first-born Son" (Matt. 1:25); as though he knew her after she gave birth to Christ. The word "till" in this text does not signify definite time but indicates indeterminate time. Sacred Scripture frequently asserts with emphasis that something was done or not done up to a certain time, as long as the issue could remain in doubt. Thus we read in Psalm 109:1: "Sit Thou at My right hand until I make Thy enemies Thy footstool." There could be some doubt whether Christ would sit at the right hand of God as long as His enemies did not seem to be subject to Him; but once we know that they are, no room for doubt could remain. Similarly there could be some doubt as to whether Joseph knew Mary before the birth of God's Son. The Evangelist took pains to remove this doubt, thus giving us to understand beyond all question that she was not known after giving birth.

Nor does the fact that Christ is called Mary's first-born give any support to the error, as though she bore other sons after Him. For in scriptural usage the son before whom no other is born is called the first-born, even though no other should follow him. This is clear from the case of the first-born sons who according to the Law were consecrated to the Lord and offered to the priests.

Again, the error of Helvidius receives no support from the Gospel narrative that certain individuals are called the brethren of Christ, as though His mother had other sons. Scripture is accustomed to apply the name brethren to all who belong to the same relationship. For example, Abraham called Lot his brother, although Lot was his nephew. In the same way Mary's nephews and other relatives are called Christ's brethren, as also are the relatives of Joseph, who was reputed to be the father of Christ.

Accordingly the Creed states: "Who was born of the Virgin Mary." And, indeed, she is called a virgin without any qualification, for she remained a virgin before the birth, at the birth, and after the birth of Christ. That there was no impairment of her virginity before and after Christ's birth, is clear from what has been said. More than that: her virginity was not violated even in the act of giving birth. Christ's body, which appeared to the disciples when the doors were closed, could by the same power come forth from the closed womb of His mother. It was not seemly that He, who was born for the purpose of restoring what was corrupt to its pristine integrity, should destroy integrity in being born.