פירוש על דברים 7:4

כי יסיר את בנך מאחרי FOR HE WILL TURN AWAY THY SON FROM FOLLOWING ME — i.e., the son of the heathen when he marries thy daughter, will turn away thy son (grandson) whom thy daughter will bear unto him from following Me. This teaches us, that thy daughter's son who is born of a heathen is termed thy son, but thy son’s son, who is born of a heathen woman, is not termed thy son, but her son, for, you see, in regard to the statement: “his daughter thou shalt not take [to thy son]" it does not add “For she will turn away thy son (grandson) from following Me" (Yevamot 23a).

כי יסיר את בנך מאחרי, “for he will lead your son astray from following Me.” The Torah does not say that the mother, i.e. the daughter-in-law of such a marriage will lead your son astray. but it speaks about the next generation, i.e. the grandson from your daughter married to the Gentile. The grandson from a Gentile daughter-in-law could not be referred to as “your son,“ as, seeing his mother is not Jewish, he is not either (compare Kidushin 68). The syntax of the verses makes clear that the Torah considers the mother pivotal in determining who is Jewish. The plain meaning of the verse then is: ”if the son of a Gentile marries your daughter he will lead your grandson born from your daughter astray” (religiously speaking, even though technically your grandson is Jewish). Her husband will train his son to engage in idolatry.

Who is born of a gentile, is considered your son, etc. Rashi is answering the question: The verse, “For he will turn your son,” seems to be giving a reason for the previous verse. But for what is it giving a reason? It cannot refer to the beginning of the verse, “Do not give your daughter to his son,” for then it should say, “For he will turn your daughter, etc.” And it cannot refer to the end of the verse, which is the simple understanding: “Nor take his daughter for your son,” and this is why it is written, “will turn your son.” This cannot be true, for it is written, “For he will turn, etc,” but it should say in the feminine form, “For she will turn.” Rather, if the son of the gentile, etc. In other words: For he will turn him away from Me, but your son’s son born of a gentile woman is not considered your son. For this reason the Torah is not concerned with him, because he is already turned away.

כי יסיר, “for he will turn away;” this is the son of a Canaanite marrying a Jewish girl, mentioned in verse three. The father of such a child will bring him up as a heathen, although legally, he is a Jew, his mother being Jewish. In the case of the child being a daughter, this will be even more so, as living as a heathen is so much easier for the children who do not have to observe all the commandments of the Jewish religion. (B’chor shor)