Lebanon is a microcosm, and an object lesson. It is a country where Christianity is on the wane. By one estimate, it was once over 70 percent Christian; today it is less than half that. Shi'ite Muslims alone probably outnumber Lebanese Christians (mostly Maronite). The decline may be greater than that. The Washington Post reported a few years ago that Lebanon has not conducted a census for about 50 years "out of concern that the evidence of Christian decline and Shi'ite Muslim advancement might fuel sectarian tension."

A similar pattern holds across the Middle East, where the Christian downfall has been dramatic. The Catholic Archbishop of Algeria, interviewed recently by the New York Times, described "the ebbing of Christianity from North Africa's shores as Islam spreads across Europe." In 1958, there were more than 700 churches in the country where St. Augustine was born and died. Now there are about 20, and they are mostly empty. "The rest have been converted into mosques or cultural centers or have been abandoned." The archbishop says Mass for a remnant of 20 people.

In The Decline of Eastern Christianity Under Islam (1997) Bat Ye'Or wrote that 13 percent of the Middle East was Christian a century ago. Today that figure may be as low as 2 percent.

Something similar is happening, although more slowly, in Europe.

The English writer G.K. Chesterton feared that if Christianity ever began to disappear, "superstition would drown out all your old rationalism and skepticism." He is often quoted as saying that when people stop believing in God "they don't believe in nothing, but in anything." But no one has been able to find that famous remark in all his copious writings. He did write something similar-that your "hard-shelled materialists [are] all balanced on the very edge of belief-of belief in almost anything."

Writing after the suicide attacks on the London Underground last year, the British historian Niall Ferguson drew attention to Chesterton's comment (and misquotation) and added that the "moral vacuum" left by de-Christianization seemed to be creating a "soft target for the religious fanaticism of others." Plainly, the rise of Islam in Europe is directly related to the fall of Christianity.

"Into the void are coming Islam and Muslims," Daniel Pipes wrote in the New York Sun two years ago. "As Christianity falters, Islam is robust, assertive and ambitious." He foresaw a time when Europe's "grand cathedrals will appear as vestiges of a prior civilization." Until they are transformed into mosques, that is.

A RECENT STUDY SHOWED that the Catholic Church in Britain is facing its greatest threat since the Reformation. Its membership is in "terminal decline," much of it recent. What Henry VIII persecuted the modern world simply ignores. The faith is withering away. Meanwhile, the Church of England has devolved into a museum. Those old cathedrals are still maintained-even admired as works of art. Increasingly, however, they are venues for music festivals. As for the Church of England's Episcopalian equivalent in the U.S., it suggests nothing so much as a wounded animal beset by carnivores.

How to explain this decline? Nothing less than a book-length response would suffice. But here are a few thoughts. Enfeebled bishops, selected precisely for their feebleness, preside over dwindling flocks. Bishops have lost all authority and few listen to their public comments, which almost always deal with material (not spiritual) concerns. Rome slumbers on, imagining that English Catholics must above all repair the breach with Canterbury, and that the way to do so is to stand for as little as possible. Diplomacy triumphs over conviction. There is no sign that the old pope, John Paul II, paid attention to the problem. Benedict XVI understands that Catholicism is in trouble in Europe, but has not yet shown that he has the courage to do anything about it.

Christianity has been under constant attack since the time of the Enlightenment and the attacks have come from within. In recent decades, mullahs and imams have hardly needed to say a word against Christianity. All the work was being done for them by critics, reformers, apostates, defectors. In some ways Muslims are actually more respectful of Christianity than the Church's internal foes. And Islam's spiritual leaders have not lost the faith, defective though it is in key areas.

Friedrich Nietzsche, the son of a Lutheran pastor, composed his nonsensical but influential work The Antichrist (better translated as "The Anti-Christian," as Walter Kaufmann noted), and proclaimed the death of God in a country still nominally Christian. His diatribes were influential not because they were cogent but because they were daring. He was thought profound merely because he had disturbed the peace. (In earlier centuries such exercises in egotism were smothered at birth.) In droves, Western intellectuals wanted to believe that they could safely defy the parson-ignore a creator perceived as more tyrannical than loving. So they disparaged the "wishful thinking" of believers, and in so doing imputed their own mindset to the faithful remnant.

The Jesuits turned to liberation theology, and many have also fallen under the homosexual spell. A succession of popes has failed to mount any effective response.

A likely ingredient in the Christian decline is increased material prosperity, which turns minds and hearts toward the things of this world. One could say that wealth makes materialists of us all. (But why hasn't there been an equivalent decline in the United States?) The Muslims of the Arab world, in contrast, have been unable to achieve anything beyond rudimentary levels of development. A material advance in the Middle East comparable to that of Western Europe possibly would undermine Islam. But why has it not already happened? The Muslims seem unable to achieve one of the most basic features of Western civilization-the rule of law. And they have remained largely frozen in a pre-medieval past. The strong rule by fear, force, and power replace law and consent, and property is insecure.

THE MOST COMMON SECULAR RESPONSE to all this is to say: What was so great about Christianity? One blogger responded to Niall Ferguson: "I don't get it. What's wrong with, say, secular humanism filling the moral vacuum? Why does he think the Christian doctrine is irreplaceable?"

It's an important question, deserving a full response. Just at the most basic level of demography, however, the secular-humanist option is not working. Sustaining a population requires each woman on average to bear 2.1 children. In the European Union, Daniel Pipes wrote, "the overall rate is one-third short, at 1.5 and falling." Should current population trends continue and immigration cease, the EU population of 375 million could fall to 275 million by 2075. Furthermore, birth rates are not lower than they already are (in France and Britain, for example, they are noticeably above the EU average) because Muslim immigrants to those countries are having more children than fallen-away Christians.

As for those who imagine that the Christian legacy is one of imperialism, racism, and inquisition, and we are better off without it-legions on the left do believe that-they will have to start thinking about what will replace it. Some are already doing so. Whittaker Chambers is worth reading on these issues, even though Islam was still dormant when he wrote Witness. The attempt to reconstruct faith without God produced Communism, he wrote. And the attempt to live without any faith at all is, I believe, impossible. If faith collapses, civilization goes with it.

---Tom Bethell is a senior editor of The American Spectator and author of the new book, The Politically Incorrect Guide to Science (Regnery Publishing). This article appears in the October 2006 issue of The American Spectator