Marx believed that, as society moved from feudalism toward capitalism, religion would inevitably decline. The nearly two centuries since Marx wrote those memorable lines have undoubtedly witnessed a decline in religious belief and practices. Even though there have been reports of renewed interest in religion among younger generations, twenty-first-century Western society has become less religious by almost any metric. One particularly interesting phenomenon emerging from this long decline witnessed in recent years is the emerging social demographic of the “spiritual but not religious”: people who explicitly identify as not belonging to any “religion,” yet who still see themselves as “spiritual.”

But what does it mean to be “spiritual but not religious”?