The case for a new, fuller understanding of what defines Judaism.

As any Jew knows, trying to define what it means to be Jewish is difficult, if not impossible. Yet still we try: over the past two decades, the number of American Jews who define themselves as secular has nearly doubled; in Israel, a country founded on secular and nationalistic notions of Judaism, the religious population has risen dramatically. Fifty-eight percent of Israeli Jews now consider themselves either traditional or religious, while just 42 percent say they’re secular.

But all these self-definitions fail to convey what Judaism truly is. Its religious aspects can be no more easily separated from its cultural or national dimensions than secular notions of Jewishness can be divorced from their religious origins. Still, a common assumption today is that Judaism began as a religion and only gradually grew into something more broad — and it’s flat wrong.

The idea was most recently given voice in the international bestseller “The Invention of the Jewish People,” by the Israeli historian and anti-Zionist Shlomo Sand, in which he argued that the idea of Jewish peoplehood was a modern invention in the service of the Zionist cause. Or as Tom Segev succinctly summarized Sand’s argument: “There was never a Jewish people, only a Jewish religion.”

The strange thing is that this has it exactly backwards: the very idea that Judaism is a religion is a distinctly modern invention. Prior to Jewish modernity — most clearly defined as the acquisition of citizenship rights for Jews, a long process that began in Europe in the late-18th century — Judaism was neither solely a religion, nor simply a matter of culture or nationality. Rather, Judaism and Jewishness were all of these at once: religion, culture and nationality.

The basic framework of organized Jewish life in the medieval and early modern periods was the local Jewish community. While a Jewish community’s existence depended on the whim of others (usually the nobility or royalty), pre-modern Jewish communities were unique in that they had a tremendous amount of political autonomy.

Each community had its own set of bylaws administered by laypersons who, among other things, elected a rabbi for the community. Rabbis in turn had jurisdiction over ritual law and also gave credence to the laws of the community as a whole.

Each community also had its own courts, as well as its own educational, health, economic and social services systems. Outside rulers gave the Jewish community responsibility to maintain law and order, and the right to punish its members in a variety of ways, including exacting fines, imprisonment and corporal punishment.

For all these reasons, it simply was not possible in a pre-modern context to conceive of Jewish religion, nationality, and what we now call culture as distinct from one another. A Jew’s religious life was defined by, though not limited to, Jewish law, which was simultaneously religious, political and cultural in nature.

It was only in the modern period that the corporate Jewish community dissolved, and with that, political agency shifted to the individual Jew, giving him the freedom to define his identity for himself.

So where did the idea that Judaism was only a religion come from? Moses Mendelssohn.

The German Jewish philosopher, born in 1729, essentially invented it. Known as the “German Socrates,” Mendelssohn thrived in both Jewish and German Enlightenment circles. Yet despite his fame, Mendelssohn, like all other Jews, had no civil rights.

When he was publicly challenged to explain why he Jews shouldn’t convert to Christianity, he argued that Judaism was wholly compatible with German Enlightenment values. But he stressed Judaism’s religious components over its corporate structure, thus giving birth to the idea that Judaism was a religion alone.

He vehemently opposed the idea that the Jewish community should retain its autonomy in matters of civil law, stressing that Jews should receive civil rights as individuals and not as a corporate entity. And he especially rejected the Jewish community’s claim, still maintained in his day, to the right to excommunicate.

It is not surprising that a century after Mendelssohn’s peak of fame, and after Jews had been granted some though not all civil rights, Rabbi Abraham Geiger, the Reform movement’s founding father, would affirm what he called Judaism’s “religious-universal” element. Though he reacted against Mendelssohn’s insistence that Jews maintain religious practices, Rabbi Geiger argued that Judaism consisted only of “spiritual achievements” because “it is precisely to its independence from political status that Judaism owes its survival.”

What is perhaps surprising is that what we today call “Orthodoxy” has as much, if not more, in common with Mendelssohn’s conception of Jewish religion than do pre-modern forms of Judaism. Despite the perception of it being deeply hidebound, Orthodox Judaism is, in other words, essentially modern. Orthodoxy’s founder was Samson Raphael Hirsch, a 19th-century German rabbi of what came to be called neo-Orthodoxy, and who stressed that a “unity of religious outlook,” and not political life, linked Jewish communal life throughout the ages.

Rabbi Hirsch never denied that non-Orthodox Jews were Jewish, but he parted company with his rabbinic predecessors in distinguishing between being Jewish and “the genuine Jew” who belonged to what he called “the true Jewish congregation.” For this reason, Rabbi Hirsch, despite his vehement criticism of liberal Judaism, made Judaism more like the Christianity of his times, much as Reform Judaism did.

The idea that Judaism was a secular, cultural identity was born further east, in the late-19th century. Rabbis Hirsch and Geiger’s ideas of Judaism as a religion made little sense in Eastern Europe, where Jews still, for the most part, did not possess individual rights. The cultural Zionist Ahad Ha’am, born in 1856 in Kiev, rejected the idea that Judaism was a religion, arguing that Jews had attempted to eliminate their communal identity for the false promise of full equality in a modern state.

As he put it in a well-known and aptly titled essay, “Slavery in Freedom”: “Do I envy these fellow Jews of mine their emancipation? I answer in all truth and sincerity: No! A thousand times No. … I have at least not sold my soul for emancipation…” Instead, he believed Jews should revive their own homeland in Palestine, and one founded on a rich Hebrew culture.

Yet while Ahad Ha’am is today often treated as a “secularist,” there can be no denying that, somewhat paradoxically, he understood religion and theology as the vital element of what he regarded as the future of Hebrew culture. Put another way, the notion of Jewish culture, or Jewish secularism, relied on religious sources. Particularly telling is how Ha’am drew inspiration from the Hebrew prophets, contending that the ethical imperative was the true meaning of prophecy and the unique contribution of Judaism to all of humanity.

But his younger contemporary Michah Josef Berdichevsky took things further, posing a critical challenge to Jewish secularism. Without God, he wondered, from where does this ethical imperative arise? And if not from God, then who has the authority to define the parameters of a culture when the sole source of the traditions on which it is built has been undermined? Thus, we see that the category of Jewish culture, like the categories of Jewish nationality and Jewish religion, is not without its own tensions and internal contradictions.

So what is the answer to that very modern question: Is Judaism a religion, a secular culture or a national identity? The answer, I think, is none and all of the above. This may not be entirely satisfying, but complex questions rarely allow for simple answers. The truth, as they say, is often inconvenient.

Leora Batnitzky is a professor and chair of the department of religion at Princeton University. She is author of the forthcoming book “How Judaism Became a Religion: An Introduction to Modern Jewish Thought,” to be published by Princeton University Press later this month. This essay is adapted from the book.