SLIGHT, bespectacled and friendly, Rabbi Itamar Tubul makes an unlikely frontiersman. But his colleague Ziv Maor, a spokesman for Israel’s chief rabbinate, argues that as head of the department of personal status and conversions, Rabbi Tubul plays as big a role in protecting the state as the Israel Defence Forces. On his desk in Jerusalem lie the testimony of a rabbi in Finland and a ketubah (marriage certificate) from Germany. Rabbi Tubul’s job is to determine whether the subjects of these documents, and many others, are Jewish.

Who is a Jew? This question is becoming ever more pressing for Jews around the world. It looks like a religious issue, but is bound up with history, Israeli politics and the rhythms of the diaspora. Addressing it means deciding whether assimilation is a mortal threat, as many Jews think, or a phenomenon to be accommodated. The struggle over the answer will shape Israel’s society, its relations with Jews elsewhere, and the size and complexion of the global Jewish community.

For Orthodox Jews like Rabbi Tubul, the solution is simple and ancient: you are a Jew if your mother is Jewish, or if your conversion to Judaism accorded with the Halacha, Jewish religious law. Gentiles might be surprised that for Jews by birth this traditional test makes no reference to faith or behaviour. Jews may be atheist (many are: apostasy is a venerable Jewish tradition) and still Jews. Joel Roth, a Conservative rabbi at the Jewish Theological Seminary in New York, likens this nativist criterion to that for American citizenship: Americans retain it regardless of their views on democracy or the constitution. Some strict rabbis even think that a child is not Jewish if born to a devout mother but from a donated gentile egg.

As some Jewish leaders privately acknowledge, this formula has uncomfortable racial undertones. Their response is that it causes no harm to others. Perhaps, but in the secular world it can be awkward. A few years ago, for example, state-funded Jewish schools in Britain were obliged to change their admissions codes after they were judged to have violated the Race Relations Act. And the halachic rules are increasingly troubling to Jews themselves.

For many Israelis, the rabbis are the problem. In a concession designed to widen support for the new state, when Israel was founded its secular rulers left matters of marriage, divorce and burial in the rabbinate’s hands. It decides who is eligible for these rites, as well as carrying them out—so would-be brides and grooms must demonstrate their Jewish credentials. Supplying the necessary documents and witnesses can be inconvenient and galling: people resent having to prove what they know to be true. Immigration has made the system seem not just irksome but unsustainable.

For example, the Ethiopian Jews who migrated to Israel in the 1980s-90s, risking their lives and losing relatives along the way, have faced persistent doubts as to whether they are properly Jewish in doctrine and descent. “I feel that I’m the Jew I want to be,” protests Fentahun Assefa-Dawit of Tebeka, an advocacy group for the 130,000-strong community. “I don’t want anyone to tell me how to be Jewish.” Western migrants, too, are sometimes doubted. The rabbinate considers some American rabbis too lax to vouch for their congregants and rejects their testimonies; it deems many overseas conversions inadequate. Many Israelis worry about the impact of such disdain on the diaspora’s political and financial backing for their state.

Israel’s time bomb

The biggest problem comes from the clashing consequences of two great ruptures in 20th-century history: the Holocaust and the collapse of the Soviet Union. Under Israel’s Law of Return, anyone who has, or whose spouse has, at least one Jewish grandparent can claim citizenship—a standard expressly modelled on the criteria for persecution under the Nazis’ Nuremberg laws of 1935. The Law of Return also recognises conversions that the rabbinate rejects. The wave of immigration from Russia in the past two decades means the discrepancy between these two standards has become glaring.

There are now several hundred thousand ex-Soviet Israelis who were Jewish enough to get in, but are not Jewish enough for the rabbis. Most are put off by the length and intellectual demands of the halachic conversion process (it doesn’t help that finished conversions are sometimes annulled for violations of Sabbath or other religious rules). Since Israel offers them no civil marriage ceremony, these Israelis and their partners go abroad to marry (as do some couples who prefer to avoid the synagogues). The population is beginning to divide into three parts: halachic Jews and Arabs, but also “others”. This tripartite split, says Yedidia Stern, a jurist at the Israel Democracy Institute, a think-tank, “is a time bomb”.

Some Israelis want to keep immigrants in the fold by making conversion easier. The response of liberals such as Ruth Calderon, a member of the Knesset for the centrist Yesh Atid party, is to try to prise apart synagogue and state. As a first step she has co-sponsored a bill that would make civil union an alternative to religious marriage. Ms Calderon, who has a PhD in the Talmud (a central Jewish text), wants to reclaim the oversight of Jewishness from the rabbis. Politicians like her, she says, are no longer willing to trade the right to pronounce on it for the votes of right-wingers in Israel’s fractured parliament.

The long-term choice for Israelis appears stark: between a different model of Jewishness or a different kind of Jewish state—in which intermarriage, hitherto regarded by Israelis as a diaspora woe, becomes, in a peculiar and unexpected way, a worry for them as well.

The fraying diaspora

In the diaspora, too, history has reframed the question of who counts as a Jew. In much of eastern Europe, communist strictures made worship perilous and observance lapsed. Even circumcision was discouraged. By the time the system imploded, lots of Jews had forgotten much of their heritage. Yet they still think of themselves as Jews.

In the West, freedom has opened its own gap between history and Halacha. The Pew Research Centre recently surveyed American Jews, who account for almost half the global total (see chart). The responses confirm that Jewishness is not thought to consist mostly in belief: 22% of American Jews described themselves as having no religion (swap “Christians” for “Jews” and the statistic becomes nonsensical). Even among the avowedly religious, two-thirds did not think it necessary to believe in God to qualify. To widespread communal alarm, Pew also found that intermarriage has rocketed and now predominates among the young. Excluding the Orthodox (about a tenth of the American total), 72% of Jews who wed since 2000 married “out”.

Hardly surprising, then, that some American rabbis are rethinking their definitions. Since 1983 the Reform movement has recognised the children of Jewish fathers—but, as for other progressive movements, blood is not enough. “Jewishness can’t only be an accident [of birth],” says Rabbi Rick Jacobs, president of the Union for Reform Judaism. For some, the extra ingredient is faith: ironically, whereas Orthodox notions of Jewishness ignore belief, more liberal denominations include it. For others, Jewishness is broader than either faith or lineage.

Rabbi Andy Bachman of Congregation Beth Elohim, a lively and ecumenical synagogue in Brooklyn, New York, argues that “an exclusive definition of Jews as a faith is a goyishe[gentile] construct”—an effort to fit the Jewish people into a recognisable gentile category. Attachment to Jewish history and culture also suffice, he thinks. In this dispensation, gentile spouses and converts are welcomed. The biblical story of Ruth, a gentile who took on her Israelite in-laws’ religion, is a favourite parable.

Rising intermarriage is also a fact of Jewish life in Australia, Canada, France and Britain, hosts of four of the world’s other main Jewish populations. Historically the British community has been smaller, quieter and more ossified than America’s; it has shrunk since the 1950s because of ageing and integration. But its mood is changing. Take JW3, a stylish new community centre in north London. For a Jewish institution in Britain, the building is “out, loud and proud”, as Raymond Simonson, its boss, puts it, with the word “Jewish” unusually conspicuous on the façade.

Mr Simonson eschews doctrinal disputes. JW3’s aim, he says, is to let visitors feel as Jewish as they want to be, regardless of their background: “We’re not asking you to unzip your flies or show us your mother’s ketubah.” One of its adult-education courses, designed in part for gentile spouses, is called “The Accidental Jew”. Cooking and the arts feature prominently. “I’m not saying you can sustain a whole Jewish identity by listening to Amy Winehouse or Neil Diamond,” Mr Simonson says, but they are one possible “entry point” to Jewishness, along with the Talmud, Israel and Jewish history.

Two futures, two more questions

Opposed though the innovators and hardliners seem, they share a basic aim: to ensure Jewish continuity. Mr Maor, of the rabbinate, says: “Our job is to fight assimilation, which has been the great enemy of Judaism for 2,000 years.” For liberals, the fraying of diaspora communities makes the old strictures anachronistic. For halachic sticklers, it shows that the liberal approach has already failed.

The ketubah: beautiful and powerful

These two attitudes imply very different futures. In one the Orthodox, with their strong retention and very high birth rates, will represent a rising share of the Jewish population (in Britain the haredim, or ultra-Orthodox, are thought to account for 40% of Jewish births). They will be increasingly segregated from the less observant, who will gradually drift away, meaning total numbers will stagnate or fall. The bonds between Israel and the diaspora could weaken. In the other scenario, Jews become more pluralistic and mutually tolerant, finding room for those whose Jewish identity wavers over the course of their lives, as these days identities tend to.

Thus following from “Who is a Jew?” is a second, equally charged question: in the future, how many Jews will there be? And a third: what is a Jew? For some, Jews are adherents of an ancient faith, with a quirky biological qualification. For others, they are something less formal: members of a dispersed civilisation distinguished by an ethical tradition and interrogatory cast of mind; by a legacy of persecution and tragic worldview (and the sense of humour that is its inverse); by certain tastes in food and culture.

For Yossie Beilin, a former Israeli minister, Jews are an extended family. He would like membership to depend on neither blood nor belief, but desire to belong. “It’s a sad joke”, he says, “that after the Holocaust we are telling people who feel Jewish that they are not.” He thinks this family should offer a purely secular conversion ceremony (“I do not want to disturb God, She has so many other things to do”). Many Jews don’t believe, he reasons, so why must converts? Mr Beilin is an outlier, but perhaps not for ever.