(Edited) As has already been shown to you, the words used in the original Greek text (
heos hou) do not translate exactly by the English word ‘until’. The semantic field is different in the two languages. There is no implication of reversal of a situation in the Greek word. The TEV translation of the United Bible Society gives the translation of this verse as, “Joseph did not know Mary BEFORE she gave birth.” The Greek usage gives no implication about what happens afterward. This article addresses the use of “until” at length using examples from within Scripture itself that make the Catholic and Orthodox point:
bringyou.to/apologetics/num27.htm
Regarding “firstborn”, this is a technical Jewish term of the child (also applies to the husbandry animals) that opens the womb - for which there were certain ceremonial functions to be carried out in dedicating and “buying back” the child (or the animal). Exodus 22:28ff and Deut. 15:19 address this. Being the child that opens the womb doesn’t mean absolutely that others were born to the mother.
Further, Matthew 1:25 tells us that Joseph did not have relations with Mary before the birth of her Son. Why not? There was no ban on intercourse during pregnancy under the Jewish Law under normal circumstances, and if he was going to have intercourse with her after the pregnancy why not before?
It was because in Jewish culture this would have meant that Joseph acknowledged biological fatherhood over Jesus. Because he refused to do this he was saying in essence that he did not accept Jesus as his biological son though he would accept him as his legal son. By refusing to consummate his marriage with Mary he was in conformity with the Jewish Law. By not having intercourse with Mary before the birth of Jesus he is acknowledging in principle that he was also forbidden to have intercourse with her thereafter. A woman found to be with child that is not her husband’s is forbidden forever more to him and to the man who impregnated her. (In fact, a woman caught in adultery is likewise penalized.)
Under Jewish, because Mary was found to be with child before she had consummated her marriage to Joseph she was forever forbidden to him. He could keep her as his wife but he was not allowed intimate relations with her. Had Mary born any children after Jesus she would have been stoned to death under the Law. Had Joseph claimed those children to be biologically his, he would have been stoned to death also. And Joseph, (if for no other reason, but we know there was one) being a “righteous man” would have been obedient to Jewish law whether or not anyone else knew about the origins of the Child.
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