My sister was baptized and has since married and had a child. My mother claims the child is Jewish, but how could that be? If Judaism is a religion, if someone leaves it, she’s no longer Jewish, right?
Logically, I would have to agree with you. If Judaism is a religion, then someone who doesn’t believe in the religion should be no longer Jewish. The reality, however, is that it doesn’t work that way.
Throughout the Tanach, we find Jews breaking every facet of their covenant with G‑d, joining and forming all sorts of idolatrous cults and heathen practices. Yet when the prophets chide them, they are called “My people, Israel.”
The Talmud focuses in particular on the precedent of a notorious character named Achan, who appears in the story of the fall of Jericho. “Israel has sinned,” exclaims G‑d. “They have transgressed My covenant that I commanded them.” Yet in the story’s narration we discover that the lone sinner is Achan, who took from the spoils of Jericho. The Talmud points out that nevertheless Achan is considered “Israel,” and remarks, “Israel, although he has sinned, is still Israel.”
The choice of precedent is poignant and the wording laden with subtle meaning: Achan has broken “My covenant that I have commanded them”—interpreted by the Talmud to mean not only one detail, but the entire covenant of Torah. Yet he remains not only a Jew, but “Israel”—the entirety of the Jewish People in a single individual.
The principle extends not only to genealogical Jews, but converts as well. In Tractate Yevamot we learn that once a person has fulfilled all the requirements of a proper conversion, he is considered “like Israel in all matters.” The Talmud explains those last words to mean that even if this convert would return to his pagan ways, “if he marries a Jewish woman, he has the same status as an apostate Jew, and they are considered married.”
Why does the Talmud choose to discuss Jewishness in terms of whether or not a marriage is valid? This is also precise: When it comes to having this Jew slaughter meat for you, or relying upon him in other areas of kosher and similar matters, his status may indeed be the same as that of a non-Jew. But those are technicalities, dependent on extraneous factors. Marriage, however, is the real test of Jewishness. Even if a non-Jew would marry a Jew with a chupah and a rabbi presiding with all the procedures “by the book,” the marriage does not have the validity of a marriage sanctified in accordance with Jewish law. Saying that “they are considered married” is the best Talmudic language available for “Yes, he is still Jewish.”
Based on the above statement of the Talmud, the Jewish Code of Law rules that a marriage between a Jewish man and a Jewish woman who “convert out” is completely valid. Therefore, their children are considered Jewish and could also marry other Jews.
Which brings us to your case, where a Jewish woman has joined another religion and married a non-Jew. In this instance, as well, since Jewishness is matrilineal, her children are considered Jewish.
Apparently, Jewishness is about neither religion nor race. Unlike a race, you can get in, but unlike religion, once you’re in you can’t get out. As with Achan, once you are a part of this people, you are the entire people. As Israel is eternal, so your bond with them is irreversible, unbreakable and eternal.