You know Jews for Jesus, the lovable San Francisco-based organization that uses the appeal of Jewish kinship to introduce Jews to “Y’shua ha Mashiach” (Jesus Christ). Its executive director is a pleasant fellow named David Brickner. After he critiqued my book, “Why the Jews Rejected Jesus,” in a Jews for Jesus publication and later graciously retracted a prominent factual error he made, we started e-mailing.

Brickner’s bio on the Jews for Jesus Web site emphasizes his distinguished Jewish lineage, calling him “a fifth-generation Jewish believer in Jesus.” That got my attention, since belief in Jesus is among the most powerful known acids on the existence of the Jewish people. When Jews accept Jesus, they marry other Christians or their children do, thus disappearing into the Christian population.

Did David Brickner’s family beat the odds? Actually, no.

According to Jewish law, a Jew is defined as someone who either a) has a Jewish mother or b) was converted by a rabbinic court. I asked Brickner about his mother. He replied a few days later with candor:

“She is not halachically Jewish,” he wrote, using the term for the body of Jewish law. “Her father was Jewish, but her mother was not. Both of my father’s parents were Jewish. My parents made aliyah many years ago, and my mother was accepted as a Jew under the Law of Return. That may not make a difference to you, but it does to me.”

But look, I pointed out, most American Jews maintain that only a Jewish mother counts in making a Jewish baby. While the Reform movement agrees with Jews for Jesus in affirming patrilineal descent, Conservative and Orthodox Jews make up 54 percent of America’s affiliated Jewish community (33 and 21 percent respectively). I wrote to Brickner:

“So when you tell Jews, ‘Hey, I’m a Jew just like you, and I believe in Y’shua!’ you are using the word ‘Jew,’ with its implications of kinship, to mean something which you know very well that most of your listeners do not understand it to mean. That’s deception.”

Brickner replied:

“I think it a bit ironic that the insult comes from you in light of your own yichus [ancestry]. Maybe there is some pathology behind your rigid declaration.”

He was referring to the fact that my own birth parents are non-Jews, as I wrote in my 1998 memoir about adoption and conversion, “The Lord Will Gather Me In.”

In exposing Brickner, am I guilty of pathological rigidity? I don’t think so, for three reasons.

First, truth in advertising: If Brickner were the head of Jews for Saving the Whales, it wouldn’t matter if he is unambiguously a Jew or not. But because his group’s whole pitch is based on the claim that lots of actual Jews believe in the Christian messiah, Brickner’s identity matters.

Second, his story beautifully illustrates the sociological pattern I mentioned earlier. Brickner points out that he has acknowledged, briefly, his non-Jewish background in a long sentimental article about his family’s Jewish roots. It’s tucked away on the Jews for Jesus Web site, if you know where to look.

We learn in the article, “It all began about 100 years ago in the Kamenky Jewish quarter of Zhitomir, Russia. My great-grandmother, Esther, daughter of Reb Levi Yitzkak Glaser, married Julius Finestone, a Jew who believed in Jesus.”

How the little Jewish girl, presumably with rabbinic approval, married a professed Christian is left unclear.

Anyway, Esther’s son Fred married a non-Jew, Ruth. Ruth’s daughter was David Brickner’s mother. Thus at least in this one branch, the Glaser family has disappeared from the eternal nation. When I asked Brickner about his own wife, the former Patti Vasaturo, with whom he has two kids, he joked that Mrs. Brickner is a “Moabite.” He was alluding to the non-Jewish ancestry of the biblical Ruth, who was a Moabite by birth.

So it goes.

Today, interestingly, there are “Messianic Jewish” communities that encourage “Jewish living.” Very nice. When I spoke at the Atlanta Jewish Community Center recently, a young guy came up afterward, introduced himself as a Messianic Jew, and told me he’d grown up as I did, in a Reform temple.

“Believing in Y’shua,” he said, “I feel more Jewish than ever.”

I looked at the wedding ring on his finger and asked if his wife is Jewish. Take one guess what he replied. For all his “feeling Jewish,” I sadly explained, he had consigned to oblivion any hope that he will have Jewish progeny.

Finally, reflecting on Brickner’s case allows us to ask why Jews for thousands of years have cared about matrilineal descent to begin with. Isn’t all this terribly dusty, hidebound, and rabbinic?

In the current issue of the journal Azure, Rabbi Meir Soloveichik explains the spiritual significance of the legal principle. Far from being “merely” rabbinic, matrilineal descent is assumed by the Bible itself. In the book of Ezra (10:2-5), it’s given as being “according to the Torah” to treat children born of non-Jewish women as outsiders to the community.

But why should the Bible care? Because Jews aspire to have a relationship with God like the one modeled for us in the intimacy of the relationship with our mother.

Writes Soloveichik, “It is because of God’s maternal relationship with Israel, Isaiah explains, that the Jewish people will never be abandoned: ‘Can a woman forget her child, refrain from having mercy on the son of her womb?’”

It is hardly surprising, then, that a Jew for Jesus should find himself unable to accept the Jewish mother as the criterion of Jewish identity. For our argument with Christianity turns upon the same two focal points that give meaning to matrilineal descent in the first place: namely, the Torah and its Author.

Christianity has, for centuries, meant giving up what is unique about the Jewish relationship with God, the relationship He framed at Mount Sinai in the eternal grammar of the Torah’s 613 commandments. The Torah over and over again affirms its own eternity as a practical obligation not to be altered in any way (Deuteronomy 13:1, 29:28, 30:11-14, etc.), a faith voided by the apostle Paul, who called Torah a “curse” from which we are “discharged” (Roman 7:6).

In return for giving up Torah, what does a Jew for Jesus get? A Jewish Christian will say: A relationship with God. Eternal reward. The truth.

But we already had those things. Some bargain.

Hey, I don’t mean to be too hard on Brickner. I like the guy.

I even feel warmly toward Jews for Jesus. In 1983, I was a high school senior taking classes at UCLA. Strolling on Bruin Walk one day I encountered a Jews for Jesus missionary named Sid who stumped me with Isaiah 53, a favorite Christian proof text. A typically ignorant product of a typical bar mitzvah education, I was stunned and scared to realize how little I understood about Judaism. The experience set me on a path to living as an Orthodox Jew.

I am convinced that, like Sid, Brickner wants to help. Nor did he set out to deceive Jews by claiming to be Jewish. It’s just that he isn’t, in fact, a Jew for Jesus. Too bad for him that a more accurate name for his organization, “Gentiles for Jesus,” doesn’t have the same ring to it.

David Klinghoffer’s book, “Why the Jews Rejected Jesus: The Turning Point in Western History” (Doubleday), was issued this month in paperback. He is a senior fellow at the Discovery Institute. His Web site is www.davidklinghoffer.com.