We can know that Our Lady remained virgin during and after the birth of our Lord by the Tradition of the Church, which includes Scripture and the Church's teaching Magisterium.
First: the word "till" (or "until") does not mean what you claim it to mean here. It describes what happened up to the time of Christ's birth, not what happened after that. We know this because it is used elsewhere in Scripture in this way. For example, in 2 Samuel 6:23, we read that "Michal the daughter of Saul had no children till the day of her death." Now, unless you believe that Michal had children after her death, you have to admit to "till/until" can mean something other than what you claim.
Second: The Church teaches and has always held that Our Lady remained a virgin after the birth of our Lord. This teaching was handed down to us orally and in writing by those who knew the Apostles personally, for example in the Protoevangelium of James, which was written around A.D. 120. Our Lady's perpetual virginity was also taught by the great Fathers and Saints, including Hilary of Poitiers, Athanasius, Epiphanius of Salamis, Jerome, Didymus the Blind, Ambrose of Milan, Augustine, Cyril of Alexandria, and many others. Even the arch-heretics Martin Luther and Ulrich Zwingli held Mary to be perpetually virgin, and those among the early Protestants who did not explicitly teach this nevertheless remained open to it. Calvin, for example, wrote
"Let us rest satisfied with this, that no just and well-grounded inference can be drawn from these words of the Evangelist, 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. It is said that Joseph knew her not till she had brought forth her first-born son: but this is limited to that very time. What took place afterwards, the historian does not inform us. Such is well known to have been the practice of the inspired writers. Certainly, no man will ever raise a question on this subject, except from curiosity; and no man will obstinately keep up the argument, except from an extreme fondness for disputation" 1.
To sum up: the dogma of the Perpetual Virginity of Mary is fundamental to orthodox Christian belief. Those who choose to reject it stray from that belief.
But he did not have sexual relations with her until her son was born. And Joseph named him Jesus.
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That is the verse that kept getting me in trouble with the nuns at Catholic school.
Poor Sr. Josephine....had a nervous breakdown middle of the year.
Douay-Rheims Bible And he knew her not till she brought forth her firstborn son: and he called his name JESUS.Now, now...That's just your private interpretation...Til does not mean til and firstborn does not mean firstborn...But yet son means son, name means name, and and means and...
It's like comedy central...Mary has to be sinless...Scriptural??? Of course not...But then you have to eliminate Jesus brothers and sisters, which IS scriptural...You have to eliminate Joseph consummating their marriage which is scriptural to keep Mary an unbiblical virgin...