George Weigel is best known for his official biography of John Paul II, “Witness to Hope,” and as a commentator on the Vatican for NBC News. But he is a major figure in his own right, among the most creative living American theologians. What is more, Mr. Weigel is a shining example of that elusive species, the Catholic intellectual.

Many non-Catholics have trouble with this notion, which sounds like an oxymoron. How can an intellectual, who by definition is undogmatic, simultaneously offer obedience to an all embracing worldview that proclaims its own infallibility? In Mr. Weigel’s case, dedication to the truth in no way conflicts with his faith.

In his remarkable little primer, “Letters to a Young Catholic,” Mr. Weigel took his readers on a series of pilgrimages to places where great Catholic personalities have struggled to reconcile this world and the next, places like the bar in London’s Fleet Street, where G.K. Chesterton used to drink. Chesterton was as worldly a scribbler as ever drained a pint of beer, but whereas the hack work of his journalist contemporaries has perished, his books and essays still glow with an insight that owes everything to his faith. What makes Catholic Christianity distinctive among religions is, in one sense, its worldliness.

For Mr. Weigel, it is not merely possible but actually essential to be a devout Catholic of scrupulous orthodoxy while also being a thinker of fierce independence. He may be the Pope’s biographer, but he is no hagiographer. Recently, Mr. Weigel raised eyebrows in the Vatican by broaching the sensitive issue of whether the Pope might retire. He pointed out that John Paul II would have been able to continue to bear witness, showing Christians and others how to face suffering and death, without being burdened with the care of a billion Catholic souls.

Mr. Weigel has also been courageous in confronting fellow Catholics on politics. He championed George W. Bush’s decision to widen the war on terror to include Iraq and other terrorist states, in the teeth of opposition from the Vatican and United States bishops. He frequently brings his scholarship to bear on clerical attempts to use Catholic social teaching to bolster left-wing policies on welfare, taxation, or the market.

He is not afraid to confront the issue of clerical abuse head-on. In his passionate polemic, “The Courage To Be a Catholic,” he argued that what had been misconstrued as a reactionary Church turning a blind eye to child abuse was in fact the consequence of a liberal hierarchy tolerating homosexual priests. On other fraught moral issues, such as abortion or stem-cell research, Mr. Weigel is among America’s most persuasive advocates of the conservative case, able to reach out beyond his own co-religionists as few others have done.

A Weigel essay often proves to be more suggestive than others’ much weightier works. His latest tract, “The Cube and the Cathedral: Europe, America, and Politics Without God,” does not disappoint. It may look like a picnic, but it is actually a sumptuous banquet, stuffed with enough ideas for several books. Even to summarize the argument is a formidable task; Mr. Weigel’s prose is so rich.

The “cube” and “cathedral” of the title refer to two great public buildings in Paris. The cube is the late Francois Mitterrand’s La Grand Arche, built to commemorate the bicentenary of the French Revolution and the Declaration of the Rights of Man. Inside this colossal marble and glass box, the great cathedral of Notre Dame would fit – and disappear.

That strange image, of the modernist cube eclipsing the medieval cathedral, encapsulates the conflict at the heart of Mr. Weigel’s book. To call it a battle between the spiritual and the secular does not do justice to its magnitude. The architectural metaphor primarily refers to the emerging “Christophobia” of the European Union – which, after a heated debate, declined to acknowledge the Judeo-Christian contribution to European civilization in its new constitution.

Mr. Weigel’s critique of Europe subtly distinguishes between the gradual process of secularization and a newly aggressive, official hostility toward religion. He believes that this Christophobia underlies a number of disturbing phenomena: European appeasement of Nazism, communism, and terrorism; anti-Americanism; a naive belief in the ability of international organizations to solve the world’s problems; a preference for bureaucracy over democracy; declining productivity; and a “depoliticization” that avoids tough decisions.

These complex issues are characterized but not explained by Robert Kagan’s well-known thesis that “Americans are from Mars and Europeans are from Venus.” Mr. Weigel’s cube-cathedral dichotomy addresses a more profound problem. He calls it “the crisis of civilizational morale.” “What is happening,” he asks, “when an entire continent, wealthier and healthier than ever before, declines to create the human future in the most elemental sense, by creating a next generation?” This question is, he argues, closely related to another: “Why did Europe have the twentieth century it did?”

Mr. Weigel is at pains to distance himself from any crude anti-Europeanism or American chauvinism. Indeed, he insists that, though American culture is emphatically not Christophobic, there is no reason to suppose its present religiosity will immunize the United States from the European disease. The crisis of morale European civilization is now undergoing may, he thinks, afflict its American offshoot soon.

Offering an erudite and persuasive account of the origins of this crisis, Mr. Weigel draws on the ideas of eminent Europeans who have borne witness to the descent of their civilization into the abyss of Auschwitz and the Gulag: the historically minded French theologian Henri de Lubac, the theologically minded historian Christopher Dawson, and the great Russian novelist Alexander Solzhenitsyn.

Having succumbed to a radically new “atheist humanism,” which first found expression in the carnage of World War I and in the totalitarian movements that arose from it, Europe, in the words of Dawson, turned itself into “a secular society that has no end beyond its own satisfaction … a cancerous growth that will ultimately destroy itself.”

Perhaps the least predictable and most valuable witness called in Mr. Weigel’s indictment of Europe is the constitutional lawyer Joseph Weiler – not a Christian at all, but an Orthodox Jew. Mr. Weiler, who coined the term “Christophobia,” identifies the 1968 generation as the driving force behind the radical secularism that has become the ruling ideology of the European Union.

By turning their back on Europe’s Christian patrimony, Mr. Weiler thinks, the disillusioned radicals of the 1960s are struggling to make sense of a world where, especially since the revolutions of 1989, they seem to have lost the plot. The church is their scapegoat for Auschwitz, for the failure of the left, and for the fact that their children are, in many cases, rejecting their values and even turning to religion.

For many readers, the most positive aspects of Mr. Weigel’s argument may also be the most problematic. He identifies Pope John Paul II as the prophet of 1989, the antithesis of Christophobic Europe, and Christianity as the driving force of European civilization. He lists about a hundred Christians – he could easily list many more – without whom Europe would not exist as we know it. And he nails his colors firmly to the papal mast, quoting John Paul’s literally apocalyptic call for Europe to regain its “religious dimension” if it is to have any future at all.

Non-Christians who sympathize with Mr. Weigel’s diagnosis may have difficulty in accepting this prescription. But nobody can deny the gravity of the crisis, particularly now that Europe’s collapse of morale has taken the form of demographic suicide.

Mr. Weigel suggests four possible scenarios: Europe reinvents itself as a godless paradise on earth; Europe muddles through; Europe reconverts to Christianity; or (the nightmare scenario) Islam inherits the hollowed-out shell of a civilization, reversing the historic defeat of the Turks at the gates of Vienna in 1683.

And what does Mr. Weigel think is most likely to happen? Like Mr. Weiler, he believes that the people of the cube and the people of the cathedral can coexist – but only if each respects the other’s values. Christians and Jews are enjoined to love their neighbor by God, but what authority do they invoke against those who refuse to acknowledge even a historical debt to the Judeo-Christian tradition? It is a truism that God and politics should be kept apart, lest religious intolerance intrude into the secular world. But politics without God is often far more intolerant and always ultimately sterile – in Europe’s case, literally so.

Mr. Weigel has written a tract for the times, but it is more than that: a mene tekel, a visionary warning of what may happen when men usurp the place of the divine. Mitterrand’s monolithic cube in Paris has far less in common with Notre Dame than with another, far more ancient building: the Tower of Babel.

Mr. Johnson writes frequently for the foreign pages of The New York Sun.