Many people today describe themselves as spiritual but not religious, for example on dating sites. I always say that I am not spiritual because I don't believe in spirits and other supernatural agents. But contemporary advocates of SBNR seem to have a looser idea of what spirituality amounts to that deserves scrutiny.
First we can look at what the rejection of religion involves. There is no agreed-upon definition of religion, but the concept can be captured by a 3-analysis that looks at standard examples, typical features, and explanations. Exemplars of religion include Christianity, Islam, Hinduism, etc. Part of what people are saying when they describe themselves as not religious is that their views are not those of any of the standard religions. More specifically, people are rejecting typical features of religion, including organized churches accompanied by rituals and social norms, along with supernatural agents such as the Christian God and Allah. In addition, people may be rejecting some of the standard explanations that religion is used for, such as the existence of the universe and the establishment of morality.
But what makes someone who is spiritual different from an atheist or agnostic who also rejects religion? Spirituality for some people seems to mean merely that they believe in ethical values such as caring about other people. But there are many ethical views that operate on rational principles and empathy without invoking spirituality. Values are emotional attitudes that can be objective if they are based on human needs, as I argue here.
Spirituality sometimes goes with a set of practices that may be reassuring and possibly healthy. Activities such as yoga and tai chi are good forms of exercise that make sense independent of any spiritual justification. Sometimes spirituality fits with rejection of modern medicine, which despite its limitations is far more likely to cure people than weird ideas about quantum healing and ineffable mind-body interactions. Evidence-based medicine is better than fuzzy wishful thinking.
Perhaps the most common basis of modern spirituality is just a mystical sense that the universe is somehow meaningful and benign, as captured in the slogan that everything happens for a reason, criticized here. In the absence of evidence for such meaningfulness, the most plausible explanation for why people are spiritual in this way is motivated inference: these beliefs contribute toward goals such as being emotionally secure. The universe is indeed a scary place, with more than a trillion stars that make our little planet and its inhabitants seem inconsequential. Religion provides reassurance that we are not as cosmically insignificant as science suggests. Moreover, benign gods such as the Christian Father appealingly offer a being looking out for us in the face of everyday difficulties such as disappointment, disease, disaster, and death. Disaffection with organized religion shifts people’s attention toward more amorphous kinds of reassurance that mystical spirituality seems to support.
However, under close scrutiny, spirituality is no better than religion at making sense of the world in ways consistent with evidence and argument. There are effective secular ways of dealing with the world and issues from the medical to the psychotherapeutic to the cosmological. Motivated inference is hard to avoid, but people can realize that mystical spirituality is no more plausible than traditional religious views. If you don’t like religion, you shouldn’t be spiritual either.