That sentiment has become a truism among social conservatives, who typically can’t explain what they mean by it. Which is why it sounds like right-wing boilerplate to many contemporary ears.

The late Harvard sociologist Carle C. Zimmerman believed it was true, but he also knew why. In 1947, he wrote a massive book to explain why latter-day Western civilization was now living through the same family crisis that presaged the fall of classical Greece and Rome. His classic “Family and Civilization,” which has just been republished in an edited version by ISI Press, is a chillingly prophetic volume that deserves a wide new audience.

In all civilizations, Dr. Zimmerman theorized, there are three basic family types. The “trustee” family is tribal and clannish, and predominates in agrarian societies. The “domestic” family model is a middle type centering on the nuclear family ensconced in fairly strong extended-family bonds; it’s found in civilizations undergoing rapid development. The final model is the “atomistic” family, which features weak bonds between and within nuclear families; it’s the type that emerges as normative in advanced civilizations.

When the Roman Empire fell in the fifth century, the strong trustee families of the barbarian tribes replaced the weak, atomistic Roman families as the foundation of society.

Churchmen believed that a social structure that broke up the ever-feuding clans and gave the individual more freedom would be better for society’s stability and spent centuries reforming the European family toward domesticity. The natalist worldview advocated by churchmen knit tightly religious faith, family loyalty and child bearing.

From the 10th century on, the domestic family model ruled Europe through its greatest cultural efflorescence. But then came the Reformation and the Enlightenment, shifting culture away from tradition and toward the individual. Thus, since the 18th century, the atomistic family has been the Western cultural norm.

Here’s the problem: Societies ruled by the atomistic family model, with its loosening of constraints on its individual members, quit having enough children to carry on. They become focused on the pleasures of the present. Eventually, these societies expire from lack of manpower, which itself is a manifestation of a lack of the will to live.

It happened to ancient Greece. It happened to ancient Rome. And it’s happening to the modern West. The sociological parallels are startling.

Immigration has kept U.S. population growing. But 60 years ago, Dr. Zimmerman predicted that when Mexico dried up as a source of immigrants, “we too will begin the grand finale of the crisis.” U.N. demographers predict that the Mexican fertility rate will drop below replacement levels by about 2020.

(To be sure, fertility rates are in free fall worldwide, but the West has a running head start. Demographically, what counts is who’s left standing when the population bottoms out. The numbers do not favor us.)

Why should expanding individual freedoms lead to demographic disaster? Because cultures that don’t organize their collective lives around the family create policies and structures that privilege autonomous individuals, at the family’s expense. Sustaining families is difficult and costly. History shows that when a culture ceases to value children above all, when traditional marriage and family structure is seen as merely an option, that culture will cease to have enough offspring to sustain itself.

In years to come, the state will attempt economic incentives, or something more draconian, to spur childbirth. Europe, which is falling off a demographic cliff, is already offering economic incentives, with scant success. Materialist measures only seem to help at the margins.

Why? Dr. Zimmerman was not religious, but he contended that the core problem was a loss of faith. Religions that lack a strong pro-fertility component don’t survive over time, he observed; nor do cultures that don’t have a powerfully natalist religion.

“Mothers will not bear the pains of childbirth nor fathers the worries of parenthood for economic rewards alone,” he wrote. “Fundamentally, people are familistic because they think it right and for no other reason.”

Dr. Zimmerman disdained his academic colleagues, who in his view denied history because the facts led them to conclusions they didn’t want to accept.

James Kurth, the distinguished Swarthmore political scientist who edited the new version of “Family and Civilization,” says that book’s publication made one of the nation’s premier sociologists a politically incorrect nonperson overnight.

Why should we read Dr. Zimmerman today? For one thing, the future isn’t fated. We might learn from history and make choices that avert the calamities that overtook Greece and Rome.

Given current trends, that appears unlikely. Therefore, the wise will recognize that the subcultures that survive the demographic collapse will be those that sacrificially embrace natalist values over materialist ones — which is to say, those whose religious convictions inspire them to have relatively large families, despite the social and financial cost.

That doesn’t mean most American Christians, who have accepted modernity’s anti-natalism. No, that means traditionalist Catholics, “full-quiver” Protestants, ultra-Orthodox Jews, pious Muslims and other believers who reject modernity’s premises.

Like it or not, the future belongs to the fecund faithful.

Does that scare you? It does Philip Longman. In his 2004 book, “The Empty Cradle,” he warns fellow secular liberals that demography is destiny and that those who want to preserve modernity must start having more children than “fundamentalists.”

Good luck with that. Given Dr. Zimmerman’s analysis, it’s hard to see how anything short of a profound, countercultural religious revival can turn this around. While we late moderns eat, drink and make merry, the Harvard scholar lamented, “very little public knowledge of the nearness, the inescapability or the seriousness of this impending crisis exists.”

Forewarned is forearmed — but only if one takes the warning seriously.

Rod Dreher is a Dallas Morning News editorial columnist.