One of the most common assumptions is that religiosity is linked to economic and technological underdevelopment. As a society gets more technologically and economically advanced, the thinking goes, religiosity naturally fades away and is replaced by a more secular worldview.

Exhibit A is usually Western Europe, which grew more secular as it grew richer (and much, much more violent) across the 19th and 20th centuries. Exhibit B is the world's most religious continent — Africa — which happens to be its poorest.

Under this view, the 21st century will be the century in which secularization spreads even further as the rest of the world catches up.

But when you look at the actual trends of religiosity across the world, what becomes apparent is actually the opposite: The 20th century was probably the high point of secularization, while the 21st century will likely be dominated by religion. The famous line by the French intellectual and politician André Malraux — "The 21st century will be religious or it will not be" — is on track to be vindicated.

First, let's dispense with the notion that there is some necessary causal link between economic and technological advancement and secularization. One need only look at South Korea, which was one of the poorest countries on the planet at the end of World War II, and is now one of the richest and most technologically advanced — indeed, on some metrics, more advanced than Western Europe or the U.S.

At the same time that South Korea experienced this astonishing growth, Christianity in the country grew from less than 1 percent of the population to about 30 percent today.

What about the rest of the world? Is it secularizing? To the contrary, religion is becoming one of the most important forces shaping the fate of most countries in the world.

Look at the former Communist bloc countries. They went from being officially atheist to experiencing a strong religious revival. It's impossible to mention Poland without mentioning the cultural importance of Catholicism there. Religion is also a common theme in any discussion of Russia, where the Orthodox Church has stepped in to provide a sense of Russian identity and become — for better or worse, given its alliance with the Putin regime — a key force shaping the country's culture.

Then there's China. While still officially atheist, it has never been wholly atheist in practice. In particular, Chinese folk religion and Buddhism never really went away, they just went underground (or, in some cases, not even underground). And one of the most noted phenomena about China is the astonishing growth of Christianity there, to the horror and dismay of the regime, which plays a game of both trying to coopt and suppress it.

By some estimates, pretty soon there will be more Christians in China than in the United States. What that will mean for the future of the country is anybody's guess, but it will certainly mean something.

Now look at Latin America. If you know one thing about religion and Latin America, it's that the entire continent has been historically dominated by the Catholic Church. If you know two things, it's that Catholicism is being strongly challenged by other forms of Christianity, particularly Evangelical and Pentecostal. That's a change in religion's favor: the kind of anticlerical secularism that featured so prominently in the continent mere decades ago isn't Catholicism's main antagonist anymore. In Latin America, the fight is between varieties of religion.

In the Middle East and the broader Arab world, the same phenomenon prevails: The most dominant cultural-religious trend of the 1950s was anti-colonial, socialist, secular pan-Arabism. That led mostly to autocracies presiding over corrupt governments, which resulted in a backlash that took the form of political Islam, which was the strongest vehicle for resistance to the jackboot of tyranny.

This religious revival is much broader than terrorism — most varieties of Islam that are growing are not extremist, even if they are robust and vociferous. We don't know what the Middle East will look in the future, but one thing is clear: It will certainly not be European-style secularism. Not long ago, a few hundred thousand Muslims made the yearly hajj pilgrimage to Mecca; today, the number is more than 2.5 million.

Wherever you look, religion is mutating, thriving, growing. Southeast Asia is as fiercely religious as ever. Same with India. Africa — this century's next superpower — is the most religious continent on the planet. In America, disaffiliation is changing the face of American religion, but at the same time, higher proportions of people today than in the 1950s declare believing in God, or having had a religious experience, or praying frequently.

And even in Western Europe, that bulwark of secularization, the main debate over national identity is inseparably linked to the question of the growth of Islam there (from both conversions and immigration). Indeed, Europe may be sowing the seeds of a Catholic revival.

Why does this matter?

It matters because theology has consequences. The post-Enlightenment secular worldview tends to treat religion as nothing more than a private hobby. It rejects out of hand the notion that people's spiritual beliefs matter in a broader context. When evolution tells us we're just genes trying to spread, when economists tell us all we do is maximize our self-interest, when psychologists tell us we just want to get laid — we become convinced that humans act on nothing but narrow material desires.

But that's just not true. As a matter of fact, human beings are spiritual beings first, with a natural orientation toward transcendent realities. More prosaically, to state the obvious, human beings make decisions partly based on how we understand our self-interest, yes, but also based on our worldviews, on our vision of what is true and good and beautiful.

Religion has been the most intense worldview-shaping phenomenon in history, and it will continue to be the most important worldview-shaping phenomenon of the 21st century.

Ignore this reality at your peril.

Editor's note: A previous version of this article mistakenly asserted that 50 percent of South Koreans are Christian.