Woodhead pointed to another finding in the Pew study: Most Western Europeans still believe in the idea of the soul. “So it’s not that we’re seeing straightforward secularization, where religion gives way to atheism and a rejection of all aspects of religion,” she said. “We’re seeing something more complex that we haven’t fully got our heads around. In Europe, it’s about people disaffiliating from the institution of the Church and the old authority figures … and moving toward a much more independent-minded, varied set of beliefs.”

The U.S. hasn’t secularized as profoundly as Europe has, and its history is crucial to understanding why. Joseph Blankholm, a professor at UC Santa Barbara who focuses on atheism and secularism, told me the Cold War was a particularly important inflection point. “The 1950s were the most religious America has ever been,” he said. “‘In God We Trust’ becomes the official national motto. ‘Under God’ is entered into the pledge of allegiance. That identity is being consciously formed by specific actors like Truman and Eisenhower, who are promoting a Christian identity at home and abroad, over against a godless communism. It’s the Christianization of America—as a Cold War tool.”

Over time, that dichotomous thinking has relaxed a bit. Now, a quarter of Americans are religiously unaffiliated, and secularism can mean a wide range of things. “There are ways of being secular that are more okay with hybridity, and there are ways of being secular that require more purity,” explained Blankholm. He cited the Humanistic Judaism movement, which sprang up in the U.S. in the 1960s and which rejects theism while still embracing Jewish history and culture, as an example of the former. “A term like spirituality can capture that hybridity.”

The Pew survey shows that 27 percent of Americans call themselves “spiritual but not religious.” Even though they’ve left organized religion behind, many still pray regularly and believe in God. This raises an issue for researchers, because it suggests their traditional measures of religiosity can no longer be trusted to accurately identify religious people. “I think people are doing things that don’t mirror Christianity sufficiently enough for our categories to continue to be as explanatory as they once were,” said Blankholm. “These categories are at their limit—they’re in some ways outmoded.”

Sahgal said she was aware of this problem, and sought to make the survey questions more granular so they would capture reality more accurately than the traditional questions alone would have done. So, for instance, the survey didn’t stop at asking respondents whether they believe in God. It drilled down further, asking whether they believe in God as described in the Bible or whether they believe in some other higher power.

As religiosity takes on forms that scramble our old understanding of that term, it’s forcing researchers to ask themselves anew what we talk about when we talk about religion.

“Those challenges are going to get worse—and they know it,” said Blankholm. “But I love that they’re developing a new vocabulary, because that’s exactly what we need.”