It is well known that according to Jewish Law, a child born of a Jewish mother is a Jew, even if the father is not Jewish. But if the mother is not Jewish, the child is not Jewish, even if the father is Jewish.
The question has been asked if a Jew who is baptized as a Christian still remains a Jew. The question can be expanded to include if a Jew converts to any other religion also -- if they still remain a Jew. The answer in the classical Orthodox Jewish sources is clear: They will always remain a Jew. The main issue in Jewish Law where this comes up is if they can contract a valid Jewish marriage with another Jew. Within Jewish Law, a Jewish marriage is only valid if it is between a Jewish man and Jewish woman. And indeed we see that even if a Jew converts to another faith, they are still able to contract a marriage with another Jew.
The source for this is in the code of Jewish Law, the Shulchan Aruch, Even Ha'ezer, 44:9 which states: "An apostate Jew that contracts a marriage, the marriage is completely valid and he would need a "Get" (writ of divorce) to dissolve the marriage. And even the child that is born from this marriage is Jewish and could contract a Jewish marriage - this is only if the child is born from a Jewish woman, even if she is an apostate. But if the woman is not Jewish, the child born is not Jewish, even if the father is not an apostate."
The commentator Beit Shmuel clarifies that even if the father is not Jewish and the mother is an apostate, the child is still Jewish. The term apostate here (Yisrael Mumar) means someone who exchanges his Judaism for another religion. That would include any process that the person did to convert out - including baptism.
So we see that according to Jewish Law, once it has been determined that a person is Jewish, either by the fact that his or her mother is Jewish, or that the person undergoes a proper conversion to Judaism, that the person will always remain Jewish, and will always be able to contract a proper Jewish marriage.
A good article that explains more of the classical sources and some of the other ramifications that could happen if a Jew converts our, such as baptism, can be found here:
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