There are several lines of attack used by Protestants who deny the perpetual virginity of Mary:
- The “brothers” and “sisters” of Jesus.
- The word “until” in Matthew 1:25.
- The word “firstborn” in Matthew 1:25.
- Arguments regarding the validity of an unconsummated marriage of Mary and Joseph.
Since you have some familiarity with the issues surrounding the first argument, start here:
**The “Brothers and Sisters” of Jesus: Anything New? **
By François Rossier
campus.udayton.edu/mary/Rossier.html
By the way, no one seriously questioned the perpetual virginity of Mary until Helvidius did it in the fourth century whereupon St. Jerome crushed him thoroughly in a written response. Luther, Calvin, and John Wesley among other early Protestants all accepted that Mary was ever-virgin. The idea that she was not is a relatively modern novelty.
LUTHER, CALVIN, AND OTHER EARLY PROTESTANTS
ON THE PERPETUAL VIRGINITY OF MARY
Martin Luther
“Christ, our Savior, was the real and natural fruit of Mary’s virginal womb…This was without the cooperation of a man, and she remained a virgin after that.”
“Christ…was the only Son of Mary, and the Virgin Mary bore no children besides Him…I am inclined to agree with those who declare that ‘brothers’ really mean ‘cousins’ here, for Holy Writ and the Jews always call cousins brothers.”
“A new lie about me is being circulated. I am supposed to have preached and written that Mary, the mother of God, was not a virgin either before or after the birth of Christ…”
“Scripture does not say or indicate that she later lost her virginity…When Matthew says that Joseph did not know Mary carnally until she had brought forth her son, it does not follow that he knew her subsequently; on the contrary, it means that he never did know her…This babble…is without justification…he has neither noticed nor paid any attention to either Scripture or the common idiom.”
John Calvin
“Helvidius displayed excessive ignorance in concluding that Mary must have had many sons, because Christ’s ‘brothers’ are sometimes mentioned.”
“The inference he [Helvidius] drew from it was, that Mary remained a virgin no longer than till her first birth, and that afterwards she had other children by her husband…No just and well-grounded inference can be drawn from these words…as to what took place after the birth of Christ. He is called ‘first-born’; but it is for the sole purpose of informing us that he was born of a virgin…What took place afterwards the historian does not inform us…No man will obstinately keep up the argument, except from an extreme fondness for disputation.”
“Under the word ‘brethren’ the Hebrews include all cousins and other relations, whatever may be the degree of affinity.”
Huldreich Zwingli
“I have never thought, still less taught, or declared publicly, anything concerning the subject of the ever Virgin Mary, Mother of our salvation, which could be considered dishonourable, impious, unworthy or evil…I believe with all my heart according to the word of holy gospel that this pure virgin bore for us the Son of God and that she remained, in the birth and after it, a pure and unsullied virgin, for eternity.”
Heinrich Bullinger
“The Virgin Mary…completely sanctified by the grace and blood of her only Son and abundantly endowed by the gift of the Holy Spirit and preferred to all…now lives happily with Christ in heaven and is called and remains ever-Virgin and Mother of God.”
John Wesley
“I believe…he [Jesus Christ] was born of the blessed Virgin, who, as well after as she brought him forth, continued a pure and unspotted virgin.”
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Let me say here that like the subject of infant baptism, the matter of Mary’s Perpetual Virginity cannot be settled definitively by either side from the scriptures alone. For you, as a possible convert to Catholicism, this means that your sister cannot use the Bible to prove Catholicism wrong. Conversely, you will not be able to “prove” her wrong using scripture, either. The argument will eventually reach a stalemate that can be summed up like this:
Scripture provides no clear answer to the questions about Mary’s Perpetual Virginity. Consequently, you may follow the tradition of your denomination and reject the idea in good faith. Likewise, I may follow the tradition of the Catholic Church in equally good faith because the Bible does not contradict either of us.
As a Catholic, of course, I will look to the historical record and to the teachings of my Church to guide me on this matter, but I don’t expect you to do so because of your adherence to sola scriptura. I have tried to explain why I feel that the Catholic view is acceptable and to remove any impediments to your understanding and agreement. I may fail to convince you, but at least you know that Catholics have a strong case and solid reasons for their belief.
In summation, if you choose to deny that Mary was ever-virgin, that’s your right. However, you cannot point to any scripture as a proof-text of your position because the Bible simply does not teach anything contrary to the Catholic belief.