For a brief moment at the end of the Cold War, it was possible to believe in the end of history. With the defeat of communism, the Western world seemed to have arrived at a final, irrevocable belief that liberal democracy was the best form of government. If 1789 ushered in an era of ideological warfare, in which nations fought primarily in order to decide how men should be governed, then 1989 brought that era to an end. Today, however, the very phrase “the end of history,” made popular by Francis Fukuyama, seems like a relic of an impossibly naïve moment. For in the post-Cold War euphoria, the political scientists forgot a truth that a novelist, Marcel Proust, enunciated long ago: Not all people living at the same time are occupying the same moment in history.

It took the West 500 years to reach our current consensus that messianic passions should be banished from secular politics. This does not mean that religious values have no role to play in public life. On the contrary, it was only by draining the theological fury from political debate that the West, and especially America, was able to harness the constructive power of faith, to make belief an ally of the secular order. America, to the confusion of many observers in postreligious Europe, is both the most religious society in the West and the most democratic.

Mark Lilla, in “The Stillborn God” (Knopf, 352 pages, $26), does not pay much attention to America, so he does not have to reckon with its usually successful reconciliation of belief and freedom. He takes his bearings, rather, from the Muslim world, where the process of desacralization has not yet taken place. Religious terrorists such as Osama bin Laden live in a premodern world, where doctrine still domineers over politics. And as the attacks of September 11, 2001, proved, when a medieval mind has access to modern technology, the results are catastrophic.

To preserve our hard-won secular dispensation, then, the West needs to remember how and why we achieved our political emancipation. Our current freedoms are not, we now know, the irrevocable product of a Hegelian march of history. They are, rather, like the fertile lands the Dutch won back from the ocean: the reward of great labor and constant vigilance.

That is the message Mr. Lilla, a professor at Columbia University and a regular contributor to the New York Review of Books, brings in “The Stillborn God.” This short book, based on lectures Mr. Lilla delivered at Oxford, is designed to introduce the general reader to one of the most important chapters in modern intellectual history. By examining a series of philosophers and theologians, beginning with Thomas Hobbes and ending with Karl Barth and Franz Rosenzweig, Mr. Lilla sketches the history of the West’s auto-emancipation from what he calls “political theology.” This focus puts Mr. LillainthetraditionofLeoStrauss, whose work centered on what he called “the theologico-political problem” — although Mr Lilla, whose approach and allegiances are different from Strauss’s, never mentions that controversial name.

This history needs study, Mr. Lilla argues, precisely because the very idea of political theology sounds strange to modern ears. We have so successfully divorced politics from religion in the Western world that we have forgotten they were once united—and are surprised to find that, in other parts of the world, they still are. “Intellectual complacency,” Mr. Lilla writes, “nursed by implicit faith in the inevitability of secularization, has blinded us to the persistence of political theology and its manifest power to shape human life at any moment.”

What defines political theology, as opposed to political philosophy or political science, is that it starts its inquiry into the best form of government by asking about God, ratherthanaboutman. Thismakes perfect sense in any culture where God stands at the center of the universe. If the world itself was made according to God’s plan, then surely human societies too should be shaped according to His will. During the Middle Ages, Christian Europe endlessly debated what God’s will entailed—above all, whether Pope or Emperor should have supreme power. (This question forms the political background to Dante’s “Divine Comedy” — Dante took the Emperor’s side.)

For Mr. Lilla, the key point about the history of Christian political theology is that this debate could not be resolved. His first chapter, “The Crisis,” points out that Christianity, unlike Islam or Judaism, was not built around a polity and did not possess a law code. It began, rather, as a faith opposed to power — the power of the Roman Empire, which crucified Jesus — and the central message of the Gospels is that the Christian does not belong in this world. When Christianity eventually obtained what Mr. Lilla calls an “accidental empire” — when Constantine made it the official religion of Rome — it was forced to deduce a political theology from this unpromising material. The result was that, throughout the Middle Ages, the ideal of Christian kingship over the world competed with the lure of monastic withdrawal from the world, and the more dangerous one of antinomian, messianic redemption of the world.

The thinker who first escaped this “labyrinth of political theology,” according to Mr. Lilla, was Thomas Hobbes. In “Leviathan,” Hobbes broke with the whole tradition of Christian political theology—not by attacking it frontally, but by quietly setting it aside. Instead of beginning his inquiry by asking what God wants for man, Hobbes started by asking what man wants from God. Human beings invent the idea of God, he argued, out of fear—the “fear of death, poverty, or other calamity” which gives man “no repose, nor pause of his anxiety, but in sleep.” We create religion in order to give ourselves the illusion of control overreality, whichisdeeplythreatening to us. But in fact, religion only makes our situation worse, because it encourages men to kill for their beliefs. Hobbes considered the religion of his day a “Kingdom of Darkness,” ruled over by ignorant men who used deception to secure their power.

His alternative was to create an entirely secular government, entrusting all political power to one sovereign. This sovereign would thus become an “earthly God,” exercising in reality the power that religion ascribes to the deity. To modern eyes, this looks like a frightening, totalitarian prospect, and subsequent thinkers, like John Locke, offered a more liberal vision of secular politics. But Hobbes deserves credit for inaugurating what Mr. Lilla calls a “GreatSeparation…severingWestern political philosophy decisively from cosmology and theology.”

In the Anglo-American world, the liberalism of Locke became the basis for a new secular democratic order. It would be refined and expanded, in time, by liberal thinkers like David Hume and John Stuart Mill, but it would maintain the principles of the Great Separation. Mr. Lilla, however, does not trace this flourishing branch of the Hobbesian lineage. If he had done so, his picture of a Great Separation would have been complicated by the fact that 19th-century England, like America today, was a liberal society with strong religious beliefs.

Instead, “The Stillborn God” turns to three Continental thinkers whose ideas about political theology were much more equivocal: Rousseau, Kant, and Hegel. With all of these philosophers, however, Mr. Lilla pays less attention to their formal political philosophy than to their deep-rooted intuitions about religious faith. Thus he focuses not on Rousseau’s “Social Contract” but on his novel “Emile,” especially the famous “Profession of Faith of a Savoyard Vicar.”

In this section, a liberal priest explains his recognizably modern approach to religious belief. Instead of accepting the doctrines of Christianity, or reading the Bible as God’s word, the Vicar believes first of all in his own conscience. “I believe all particular religions are good when one serves God usefully in them,” Rousseau writes; “the essential cult is that of the heart.” In this way, Rousseau hopes to recover the spiritual advantages of religion without resurrecting its fanatical dogmatism. A vaguely defined goodness takes the place of a demanding God.

Kant, who was deeply influenced by Rousseau, had a similarly pragmatic approach to religion. He came to believe not that we should be moral because God commanded it but rather that we should believe in God because morality would benefit from His existence. Finally, Kant even endorsed the existence of national churches—again, on the practical grounds that only religious community allows people to work together for moral ends. In the next generation, Hegel, with his belief in a constantly progressive History, went still further: for him, religion is valid because it is part of a nation’s historical culture, a means of its self-knowledge and self-development. To Kant, one might say, Christianity was right not because it was true, but because it was necessary; to Hegel, it was right merely because it existed.

In his book’s last section, Mr. Lilla turns to the unhappy end-point of this line of thinking. After World War I, he writes, the “crisis” theology of Karl Barth and the Jewish messianism of Franz Rosenzweig demonstrated that the dangers of religious thinking were alive and well. These theologians’ language of “decision” and “redemption” chimed all too neatly with the ideological fanaticism of the Weimar years, and maybe even paved the way for Nazism and communism. Barth’s colleague Friedrich Gogarten, Mr. Lilla points out, embraced the so-called “German Christian” movement, which sought to subordinate the Protestant churches to the Nazi Party.

This concluding chapter of “The Stillborn God” is the most unconventional, and perhaps for that reason the most arguable as well. It seems strange to end a history of modern political thought by decrying the errors of 20th-century theologians like Barth, who for all their genius remain footnotes to the history of our times. For it is surely the case that the messianic energies that modern Europe had seemed to banish made their return, not in the relatively insignificant guise of post-liberal German theology, but in the historically crucial form of Marxism. And the absence of Marx and Marxism from “The Stillborn God” leaves its central argument unresolved.

For the phenomenon of communism — and, in another way, that of fascism as well — suggests that the totalizing, fanatical tendencies of the human mind cannot be strictly identified with religious belief. Banish Christianity, and you may have no more wars of religion; but the 20th century’s wars of ideology were even bloodier. It may even be the case, as the faithful continue to argue, that only our religious traditions preserve that reverence for human life which is the prerequisite of liberalism.

Yet “The Stillborn God” still has a timely lesson to deliver. All religions, Mr. Lilla reminds us, harbor a potentially catastrophic millenarian impulse. Give them an inch of credence, and they may end up claiming a mile of authority. “The Great Separation,” he writes, “was never a fait accompli”; it requires renewed commitment in every generation, and especially our own.