Their liberal counterparts don’t fare much better: “Liberal Protestant churches, which have famously lax requirements about praxis, belief, and personal investment, therefore often end up having a lot of half-committed believers in their pews,” writes Connor Wood, a PhD candidate in religious studies at Boston University. “The parishioners sitting next to them can sense that the social fabric of their church isn’t particularly robust, which deters them from investing further in the collective.”

But not belonging to a religious institution doesn’t mean you don’t have a cultural attachment to your religious history. Just look at the more familiar concept of cultural Judaism. Among younger American Jews, cultural ties are increasingly the basis of their connection to their faith; 32 percent of Jewish American millennials told a Pew survey in 2013 that their Jewish identity is based on ancestral, ethnic and cultural connections rather than religious ones. Rabbi Miriam Jerris of the Society of Humanistic Judaism says cultural Jews and cultural Christians who celebrate their religious traditions have a lot in common (though they’re not completely analogous, because Judaism has a long history as both a religion and an ethnicity): “These people are looking for communities and for memories from their background, but they want to do it in an intellectually consistent way.”

The decline in religious affiliation has Christian churches worried, but they aren’t alone in their concern. “The big question I have is: Where are they going in times of crisis? Where do they go to celebrate life’s joys?” says Chris Stedman, the executive director of the Yale Humanist Community and the author of “Faitheist,” which argues that atheists can find common ground with born-again Christians. “There are people looking for centralized ways to organize their communities around moral identities that they might be missing.”