How fitness classes provide the meaning that religion once did.

Institutions like CrossFit and SoulCycle are offering their students more than just a chance to lose weight or tone up. They function, ter Kuile argues, like religions.

“People come because they want to lose weight or gain muscle strength, but they stay for the community,” he said. “It’s really the relationships that keep them coming back.”

Of course, these spaces are themselves defined not just by their spiritual role but by their economic one. At up to $40 a class, places like SoulCycle and CrossFit often cater to a particular demographic: urban millennials with high-paying jobs and disposable incomes, the same group that tends to identify as religiously unaffiliated.