A Jew who believes in Jesus is considered to have denied the very basic tenets of Judaism. All Jewish groups agree that this is outside the realm of acceptability.

I suggest you read, "You Take Jesus, I'll Take G-d," or “The Real Messiah” –- both available in any Jewish bookstore or www.eichlers.com.

These books will explain Jewish great-great-great... grandparents who lived at the time of Jesus and who knew Jesus refused to accept him.

It will also explain why all Jewish ancestors continued to refuse to accept him -- even at the point of death, whether it was death by being thrown to the lions, or burned at the stake in the Inquisition, or death by pogrom.

It will also explain what a sad, despairing tragedy it is that some have abandoned their great heritage by believing in Jesus. Jews are generally are turned off by Christianity, even though they may be completely disconnected from traditional Judaism. Given this, some missionaries got the idea to try a backdoor tactic. They invented "Jews for Jesus," which uses a whole lexicon of Jewish-sounding buzz words in order to make Jesus more palatable to Jews.

For example, members of Jews for Jesus don't to church, they go to a "Messianic Synagogue." Prayer is not held on Sunday, but on Saturday, the Jewish Sabbath. They say that by accepting JC, you're not converting to Christianity, you're instead becoming "a fulfilled Jew." The New Testament is called "Brit Chadasha" (Hebrew for New Covenant). It's not the cross, it's "the tree." Not baptism, but "the mikveh." Not a communion wafer, but "matzah." Congregants wear a tallit and kipa, and bring a Torah scroll out of the Holy Ark - just like every other synagogue. After all, they proudly proclaim, Jesus himself was a Jew!

Having said this, you should note the following: In the book of Joshua 7:11, G-d declares that "Israel has sinned" due to a severe transgression done by the nation. The Talmud (Sanhedrin 44a) explains: Even though Israel has sinned, God still calls they by the name 'Israel.' In other words, they are still considered Jews. And that is how we know that a Jew is always a Jew no matter what.

Furthermore, a conversion to another religion is ineffective. A person is always Jewish - regardless of whether they reject their heritage, ignore it, or practice another religion.