Now this refutes also the false interpretation which some have drawn from the words of Matthew, where he says, “Before they came together she was found to be with child.” They interpret this as though the evangelist meant to say, “Later she came together with Joseph like any other wife and lay with him, but before this occurred she was with child apart from Joseph,” etc. Again, when he says, “And Joseph knew her not until she brought forth her first-born son” [Matt. 1:25], they interpret it as though the evangelist meant to say that he knew her, but not before she had brought forth her first-born son. This was the view of Helvidius which was refuted by Jerome. Such carnal interpretations miss the meaning and purpose of the evangelist.
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Therefore, one cannot from these words [Matt. 1:18, 25] conclude that Mary, after the birth of Christ, became a wife in the usual sense; it is therefore neither to be asserted nor believed. All the words are merely indicative of the marvelous fact that she was with child and gave birth before she had lain with a man. The form of expression used by Matthew is the common idiom, as if I were to say, “Pharaoh believed not Moses, until he was drowned in the Red Sea.” Here it does not follow that Pharaoh believed later, after he had drowned; on the contrary, it means that he never did believe. Similarly when Matthew [1:25] 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. Again, the Red Sea overwhelmed Pharaoh before he got across. Here too it does not follow that Pharaoh got across later, after the Red Sea had overwhelmed him, but rather that he did not get across at all. In like manner, when Matthew [1:18] says, “She was found to be with child before they came together,” it does not follow that Mary subsequently lay with Joseph, but rather that she did not lie with him. Elsewhere in Scripture the same manner of speech is employed. Psalm 110[:1] reads, “God says to my Lord: ‘Sit at my right hand, till I make your enemies your footstool.’” Here it does not follow that Christ does not continue to sit there after his enemies are placed beneath his feet. Again, in Genesis 28[:15], “I will not leave you until I have done all that of which I have spoken to you.” Here God did not leave him after the fulfilment had taken place. Again, in Isaiah 42[:4], “He shall not be sad, nor troublesome, till he has established justice in the earth.” There are many more similar expressions, so that this babble of Helvidius is without justification; in addition, he has neither noticed nor paid any attention to either Scripture or the common idiom. -Luther (“That Jesus Christ Was Born a Jew,” pp. 211-13)