Two tectonic shifts have produced the gap that fundamentalism fills. One concerns the question that has dominated the sociology of religion for more than a century: Will faith decline as modernity advances? The great thinkers of another era — Karl Marx, Émile Durkheim and Max Weber — believed that in one way or another it would. Today’s leading sociologists point to Jerry Falwell and Osama bin Laden to claim that it will not. Roy stands with yesterday’s giants. It is true, he concedes, that conservative religion is growing. But any talk of a religious revival is “an optical illusion.” Religion, he writes, “is both more visible and at the same time frequently in decline.” It cedes so much to the secular world that it can no longer offer a transcendental alternative to it.

We are, in addition, witnessing the severing of religion from the cultures within which it was once embedded. Religion and culture have long existed in an uneasy embrace. Catholicism is presumably a universal faith, yet long before the reforms of Vatican II allowed Mass to be celebrated in the vernacular, Brazilian Catholicism owed as much to its South American roots as Polish Catholicism did to its Eastern European ones. Islam sought to conquer the world, or as much of it as it could, yet it was intimately connected to the Arab culture in which it was born. The only reason we do not find the term “secular Jew” puzzling is because we appreciate that Judaism is both an ethnic and a religious category. Much the same can be said for many of the other world religions, including Hinduism and Buddhism.

If religion is in decline in the modern world, Roy argues, so is culture. On the one hand, we have multiculturalism, celebrations of diversity that somehow wind up making all cultures look and feel alike. More important, we face globalization, today’s true universal faith, which subjects all local customs to the laws of the market. Under the influence of both, religion loses whatever affinities it may once have had with the cultures that sustained it. Jakarta, the capital of the world’s largest Muslim country, lies some 5,000 miles from the holy city of Mecca, and even Mecca, Roy argues, has lost much of its specifically Arab character.

Japanese pilgrims travel to San Francisco to find a more vibrant Buddhist culture than Tokyo can offer. Mormons, no longer confined to Utah, find adherents in Manila. Even the Christian Orthodox churches that are defined by their countries of origin lose their ethnic character; significant numbers of adherents to Orthodoxy in France have refused to come under the authority of the Russian patriarch.

Culture’s stickiest component is language. When the word of God was literally expressed in words, language inhibited religious expansion; preachers could speak only to those who understood what they were saying. With the rise of Pentecostalism, one of the fastest-growing religions in the world, followers are encouraged to speak in tongues, which requires no language at all. Meanwhile television and the Internet contribute to fundamentalism’s appeal; both make it possible for Egyptian imams (preaching no doubt in “globish,” the pidgin form of English that emerges wherever globalization takes root) to reach their followers in Europe. One does not know whether to be in awe of faith’s capacity to adapt or distraught by the hodgepodge that enables it to do so. “The Holy Spirit,” Roy writes, “is anywhere and everywhere. There is no need to have a real rock on which to build the Church.” God only knows what St. Peter would make of that.