Is American religiosity on a downward spiral? Yes and no.

Religious behavior is on the decline among Millennials – even though a growing number say they believe in an afterlife.

Faith in America may be more complicated than expected.

A new study has added a wrinkle to the current understanding of American religious life, which holds that institution-weary Americans are leaving church to find solace in personalized spirituality. The study suggests that measures of personal religiosity are declining alongside church attendance.

"This decline can’t be explained by distrust in institutions, because things like prayer have also gone down," says Ryne Sherman of Florida Atlantic University, a psychology professor and researcher on this study. "If anything, there’s a higher percentage of people saying they’re not at all spiritual today."

One out of five Millennials say they are "not spiritual at all," says Jean Twenge, the lead researcher from San Diego State University and author of "Generation Me," which makes them the least religious generation in the last five decades.

"Religious involvement stays fairly steady during the 1970s and 1980s and then starts to decline in the mid-1990s, with declines in private practice (prayer, belief in God) beginning in the mid-2000s," Dr. Twenge said in an email.

The data goes back to 1972 and was collected via the University of Chicago's General Social Survey; researchers believe religious affiliation was high prior to that based on other records, but the data suggests an overall decline, rather than a cyclical one, since the 1970s, Twenge says.

"Nearly a third of Millennials were secular, not merely in religious affiliation, but also in belief in God, religiosity, and religious service attendance, many more than Boomers and Generation X’ers at the same age," researchers wrote in the study, published Monday in the journal Sage Open. "Eight times more 18- to 29-year-olds never prayed in 2014 versus the early 1980s."

The Evangelical movement may have "held off" the decline, particularly in the South, but the data points to a steady decline in religious behavior since at least the 1970s, Twenge says. The habit of identifying as spiritual rather than religious is unchanged among older generations, but young adults are using it less and less.

"A growing minority of Americans are truly secular, not religious and not spiritual," Twenge says.

This data shows that spirituality is not replacing mainstream religious observance, as many have suggested, but that a divide may be growing, numerically at least, between the faithful and the nonreligious, as Mary Beth McCauley wrote for The Christian Science Monitor:

Some experts argue that the US isn’t becoming more secular as much as it’s becoming more devout – a country with fewer followers but ones who are more serious about their faith. 

“There’s a greater willingness now to say ‘I’m not religious,’” says Christian Smith, director of the Center for the Study of Religion and Society at the University of Notre Dame and co-principal investigator of the noted National Study of Youth and Religion. As a result, he adds, “for people who do continue to practice religion, [their communities] tend to be made up of the seriously committed, not just those swept along by obligation.”

Among those whose say they're not religious, the data does not suggest a total lack of faith in religious teachings. The most surprising finding, Dr. Sherman says, was that Americans' belief in an afterlife continues to thrive. Younger Americans now believe in some sort of heaven at a higher rate than did previous generations. Americans' decreasing attention to religion in this life has an inverse impact on their concern for it in the next.

The contrast points to a rise in individualism, Sherman says, or even entitlement, as the idea of receiving something for nothing trickles through American thought.

"There’s still this idea that even if I don’t pray, even if I don’t believe in God, there’s still going to be something in the afterlife for me," Sherman says.

In an ironic twist on Pascal's Wager – the argument that a person is safer to believe in God because a believer has nothing to lose and much to gain in the afterlife – Americans increasingly believe that their declining spiritual activity in this life will not affect them in a life to come.

"It's sort of an attitude that if there is an afterlife, then God won’t really care if I pray or if I go to church," Sherman says.