Ask someone why they believe what they believe and they'll usually describe a search: scripture, prayer, a moment of certainty. The data tells a plainer story. Religion tracks birthplace and upbringing more reliably than almost anything else about a person, and once you're in, staying has more to do with how it felt as a kid than with what turned out to be true.
Pew's 2020 survey of 201 countries and territories found that most of the planet lives in places dominated by a single religious category, not a marketplace of competing options.
Yemen, Afghanistan, and Somalia are each 99.8% or more Muslim. Timor-Leste and Moldova are almost entirely Christian. If you were born in any of these, your religion was decided before you could talk.
Singapore is the world's closest approximation to an even mix: 31% Buddhist, 20% unaffiliated, 19% Christian, 16% Muslim, 5% Hindu. No country anywhere scores a perfect spread.
Some religions aren't just regionally common, they're regionally almost total. Christianity is the outlier here, the most spread-out major religion on Earth, and it still needs a dozen countries to reach barely half its own adherents.
The Muslim figure is the sharpest: six countries hold 52% of the world's roughly 2 billion Muslims, and those six countries make up only 30% of the world's population. That's concentration well beyond what population size alone would predict.
Globally, 91% of adults ages 18–54 still hold the religion, or lack of one, they were raised in. Retention varies by tradition, and the pattern below is consistent whether you look at the world or just the United States.
Among Americans who had a positive religious experience as children, 84% still identify with that religion as adults. Among those whose experience was negative, 69% now identify with no religion at all. The determining factor Pew found wasn't doctrine, it was how it felt growing up.
"How thoughtful of God to arrange matters so that, wherever you happen to be born, the local religion always turns out to be the true one."