Matrilineal Descent
There is a very difficult verse that requires explanation based on the Talmud in Kiddushin. “When your son will be removed from Me, and he will worship foreign gods, My anger will be kindled against you, and you will be destroyed quickly.”
The previous verse warns against intermarriage where you shall not give your daughter to his son, nor shall you take his daughter for your son.
What is interesting to note is that this verse (Deuteronomy 7:4) speaks of “your son” being removed from Hashem.
The surprising explanation of the Talmud is that this verse is directed towards the grandfather of this son. He is removed from Hashem because his father is not Jewish, but only his mother is. This teaches that if someone has a Jewish mother, he is still a Jew, (your son.) This implies that if the mother was not Jewish, it is her son and not your son.
Nevertheless, Hashem is not pleased with the grandfather who did not prevent his daughter from intermarrying. The result is going to be that it will be difficult to keep this young man in the fold.
The influence of his non-Jewish father might be too great to bring him back.
This verse is very timely to our state of affairs. But it is the source in the Torah for matrilineal descent. A child is Jewish, only if his mother is Jewish.