This is an important question . . . and all too relevant for very many Jews today, who either fall prey to the many attempts by Christian missionaries (such as the nefarious “Jews for Jesus”) to ensnare Jewish souls, or those who are, most unfortunately, born into an intermarriage, and are thus presented with a choice as to their religious identity.
The question presupposes a view of the Jewish view of religious identity, that is not often properly understood.
Most people are accustomed to seeing Judaism, or Christianity, or Islam, or other faith systems as simply a religion; and thus it is relevant only to their spiritual lives. It is not self-definitional. Ask a typical neighbor “Who are you?”, and they might respond, “My profession is a plumber, my nationality is American, my race is Anglo-Saxon, and my religion is Christian”. That may accurately reflect the position of the said Christian person.
A Jew, however, has a much different relationship with his Judaism. If a Jewish person, even one who has, Heaven forfend, accepted Christianity as their faith, would be asked the same question, he might accurately respond “My profession is a plumber, my nationality is Jewish American, my race is Jewish, and my religion is Christian”.
What I am getting at is that Judaism, unlike other faith systems, is not only a religion, but also a race, or in fact a nation. We are Am Yisrael – the Nation of Israel. We are Jews whether we are fully practicing religious Jews, whether we are Orthodox, Conservative, Reform, or Reconstructionist Jews; whether we practice Christianity, Hare Krishna, or even if we profess no religion at all – we are still just as Jewish as the most devoutly religious Orthodox Jew. Even one who publicly renounces their Jewish faith and turns their back on everything the Torah teaches, remains a Jew. “Even a deeply sinful Jew, remains a Jew” (Talmud Eruvin 19a). There is no escaping one’s Jewish identity.
In fact, Jewish legend has it that there was a Jewish Pope, no less. Whether or not this is true, it is clear that under Halacha, a Pope by Jewish birth would be considered Jewish.
Implicit in the Talmudic statement, however, is that though the person who has rejected Judaism remains a Jew, they are considered a sinful Jew, who will ultimately have to answer before the Almighty for their choices. A Jew who engages in such heretical behavior would be referred to in Halacha as a “Yisrael Mumar”, roughly translated as a Renegade Jew. The Rabbis of the Talmud ruled that we would not count such a person to a minyan, and in other significant ways we deny them the privileges of Jewish identity. You might have even heard of a time where the family would sit Shiva for a family member who “converted” to another religion. But that does not mean that the person is, in fact, no longer Jewish. Rather, they are no subject to strong legal and public sanction for their actions, as a disincentive to others to follow in their footsteps.
While the above paragraph sounds harsh, it is clear beyond doubt that most modern Jews who fall prey to the those groups missionizing to Jews are woefully ignorant not only of Jewish practice and teaching, but of the long history of Jews being forced to convert to Christianity at the point of a sword or guillotine… and thus will be judged with mercy by God for their choices. They are attracted by very methodically trained people who are skilled at hiding their true motives and distorting much Biblical material regarding the identity of the Messiah. It is often the fault of the Jewish community that has not provided as welcoming and loving an environment in its synagogues as can be found in so called Messianic Temples. It is crucial that we find ways to present the true teaching of the Torah in engaging and spiritually satisfying ways, so that searching Jews will realize that there is all that they hoped for and more within their own tradition. Rather than condemn such lost souls, we would do all in our power to help such Jews reclaim the spirit of their nation.
In summary, even though accepting another religion is a very serious sin, the person remains Jewish, and is of course welcome back to Jewish practice with open arms. One only hopes that the person does not intermarry and/or totally assimilate, for then they or their progeny will be totally lost to the Jewish people.
I implore any Jewish person considering adoption of the Christian faith to first study the material available at
http://www.jewsforjudaism.org/, or at http://www.outreachjudaism.org/.