Even without its subtitle, there could be no doubt about which group is being discussed in Religion or Ethnicity?: Jewish Identities in Evolution, the new collection of essays edited by Zvi Gitelman. No other people occupies precisely the same ambiguous position between religion and ethnicity as the Jews, or has to wrestle with the many anomalies that result. Historically, for instance, all Greeks belong to the Greek Orthodox church; yet a Greek who converted to, say, Lutheranism could still indisputably describe himself as Greek. On the other hand, if you were born Lutheran but decide as an adult that you no longer believe in any part of the Lutheran faith, you are obviously not a Lutheran anymore.But what about a Jew who stops going to synagogue and denies the existence of God? Clearly he is no longer a practitioner of Judaism, but does he lose the right to describe himself as a Jew? If so, where does that leave the large secular population of Israel, or the Yiddish socialists of the early 20th century—not to mention the many nonobservant American Jews who take pride in being “culturally Jewish”? Yet what if, instead of simply abandoning the synagogue, a Jew declares that he believes in Jesus and starts going to church? Doesn’t that act make him a Christian, with no further claim to membership in the Jewish people? If it doesn’t, could anything?These questions could not be more vital to contemporary Jewish life, which is why Religion or Ethnicity is the rare collection of academic papers that deserves a general readership. Gitelman has assembled an unusually eloquent and thoughtful group of contributors, who address the book’s topic from a variety of angles—historical, literary, political, even statistical. They look at the curricula of Yiddish secular schools in the 1920s, the anti-rabbinical polemics of 17th-century Dutch Sephardim, opinion polls from Israel and the former Soviet Union, and anthologies of Jewish American literature, among other subjects. And if they don’t offer any firm answers to the title question, at least they show that it has been part of Jewish life for a very long time.In the ancient world, Yaron Z. Eliav writes in “Secularism and Rabbis in Antiquity,” Judaism was defined by a “shared historical heritage…. Jews identified themselves and were perceived by their Gentile neighbors as the descendants of Abraham, Isaac, and Jacob, members of a nation who had been enslaved in Egypt, taken out of bondage with signs and wonders, received the Torah at Sinai, and whose twelve tribes had inherited the land of Canaan.” Yet this historical identity was compatible with any number of different practices and even beliefs.“It would be unimaginable,” Eliav writes, “for a present-day haredi rabbi to attend a Roman bathhouse,” but in the Second Temple period all kinds of Jews participated in the life of the gymnasium, with its “nudity, sports, and hedonistic fixation on the human body.” Jews sacrificed to God at the Temple in Jerusalem, but also made offerings to the pagan gods at Roman temples, as a standard part of civic life. In the same way, “God-fearing” pagans are recorded as having built Jewish synagogues. Even the first Christians still considered themselves to be Jews—as Eliav points out, the Gospels “confirm the centrality of the mitsvot” in Jesus’s world. Before the development of halacha, Judaism was not so much a religion as “a diversified and porous continuum” of identity.It was the codification of Jewish law in the Mishnah and Talmud, and the segregation of Jews into self-governing communities in Christian Europe, that simplified the question of religion and ethnicity. From the fall of Rome until the 18th century, most European Jews lived in a wholly Jewish world, where it made no sense to separate Jewish identity from Jewish belief and Jewish practice. Yet this does not mean, as Calvin Goldscheider cleverly argues in “Judaism and Community in American Life,” that the world of the shtetl was in any straightforward sense more observant than our own.We are all familiar with polls that show declining measures of Jewish observance in 21st-century America. But what if, Goldscheider asks, you conducted the same kind of survey in 18th-century Poland? You would find that “almost no women attend synagogue services (except in a few large cities) and then only a few times a year. Few young boys past the age of Bar Mitzvah, and even fewer young girls of any age, have any Jewish education. Neither do their parents. Many men do not attend services regularly because they do not have a quorum of ten adult men.” Poverty and ignorance could take just as great a toll on Jewish practice as affluence and indifference do today.The difference, of course, was that even when they did not or could not pray, study, or observe the holidays, Eastern European Jews were in no doubt that they were Jews because they were believers in Judaism, and vice versa. It was not until the French Revolution opened up the tortuous path of emancipation in Central and Western Europe that many Jews had the option, or the incentive, to define Jewishness in different ways. As Scott Spector writes in “Beyond Assimilation,” one of the most theoretical essays in the book, now “Jewish identity was not a starting point … or a stable entity that could be taken for granted, but rather a problematic.”Notoriously, for German and German-speaking Jews, it was a highly problematic problematic indeed. As Todd Endelman shows in “Jewish Self-Identification and Belonging,” there were several different ways of defining and measuring emancipation, which didn’t necessarily go together. Thus many German Jews were ignorant of Judaism and completely devoted to German language and culture, yet had almost no social contacts with actual Germans; they were acculturated but hardly integrated into German society. Gershom Scholem’s father was an ardent German patriot, yet “no Christian ever set foot in our home,” he recalled. Even Jews who converted to Christianity associated almost exclusively with Jews and other converts. These kinds of contradictions naturally made many German Jews unnaturally self-conscious about their identity. “What have I in common with Jews?” Franz Kafka wrote. “I have hardly anything in common with myself.”By the early 20th century, the stubborn failures of assimilation gave rise to new ideological visions of Jewishness. In different ways, Zionists and Yiddish socialists tried to instill Jewishness with positive meaning, yet without returning to traditional Judaism. In his essay on “Modern Hebrew Literature,” Shachar Pinsker shows how Chaim Nachman Bialik tried to reimagine Jewish religious texts as a kind of secular literature, sifting the Talmud for the Agadah—the narratives and parables he considered the true “folk literature of the Jews.” “The problem,” he complained, “is in the fact that the Agadah is within the legal, halachic texts, annexed to it like an appendix.”Wrenching these stories out of their original religious context, Bialik wrote, was like finding “fragments of stones” that could be “joined into layers, layers into walls, a complete fortress in which everything is arranged and installed in its proper place, restoring the ruined palace to its original glory.” Yet in the end, Pinsker argues, Bialik realized that it was impossible to detach Jewishness from Judaism so neatly. In a late poem, Bialik described his work as “digging in graves of people and ruins of spirit/And nothing remains with me and nothing is saved.”Bialik’s dilemma has never really been solved, as several contributors show in essays about the uncertain place of Judaism in the modern State of Israel. Charles S. Liebman and Yaacov Yadgar use polling data to investigate the relationship of hilonim—as secular Israelis are known—and masortim—as the mainly Mizrahi “traditionalists” describe themselves—to Judaism. Long before 1948 there was a strong anti-religious component to Zionism, and nearly half of Israelis describe themselves as non-religious. Ashkenazim make up the majority of these, thanks in part to the recent influx of non-observant Soviet Jews. Yet even among the hilonim, Liebman and Yadgar find, more than half have mezuzahs in every room of their house, and fully 84 percent want Israel to remain a Jewish state.Liebman and Yadgar, like other contributors to the volume, are pessimistic that purely secular Jewishness can thrive for long. Without divine commandment or obligation, they seem to fear, Jewishness will eventually dissipate through assimilation, intermarriage, or simple indifference. “Reform Judaism,” Gitelman writes contemptuously in his conclusion to the book, is “a default position of Jewishness, the last stop on the way out of Jewishness.” I am not sure that this kind of anxiety is justified, however. In the modern world, all identities are partly voluntary and constructed, and a Jewishness we choose—however strictly or loosely we define it—is at least as valid and honorable as one we inherit. The simple fact that we still ask what Jewishness means is itself a sign that it continues to matter. Or, as Gitelman puts it, “As long as significant numbers of people debate the issue, the survival of Jewishness is assured.”Adam Kirsch is a contributing editor to Tablet Magazine and the author of Benjamin Disraeli, a biography in the Nextbook Press Jewish Encounters book series.Adam Kirsch is a poet and literary critic, whose books include The People and the Books: 18 Classics of Jewish Literature.