Unbelief is on the uptick. People who check "None" for their religious affiliation are now nearly one in five Americans (19 percent), the highest ever documented, according to the Pew Center for the People and the Press.
The rapid rise of Nones -- including atheists, agnostics and those who say they believe "nothing in particular" -- defies the usually glacial rate of change in spiritual identity.
Barry Kosmin, co-author of three American Religious Identification Surveys, theorizes why None has become the "default category." He says, "Young people are resistant to the authority of institutional religion, older people are turned off by the politicization of religion, and people are simply less into theology than ever before."
Kosmin's surveys were the first to brand the Nones in 1990 when they were 6 percent of U.S. adults. By the 2008 survey, Nones were up to 15 percent. By 2010, another survey, the biannual General Social Survey, bumped the number to 18 percent.
Meanwhile, the Roman Catholic Church, the nation's largest religious denomination, the Southern Baptist Convention, Methodists and Lutherans, all show membership flat or inching downward, according to the 2012 Yearbook of American & Canadian Churches.
The 19 percent count is based on aggregated surveys of 19,377 people conducted by the Pew Research Center throughout 2011.
How high the Nones numbers might go depends on demographics, says Mark Chaves, professor of sociology, religion and divinity at Duke University, an expert on the General Social Survey.
Two forces could hold Nones' numbers down. First, they are disproportionately young, often single, and highly educated -- all groups with a low birth rate. Second, the number of believers who immigrate to the U.S. from particularly religious nations, such as Catholics from Mexico, fluctuates with government policies and economic issues, Chaves says.
But the chief way the category grows is by "switchers." A 2009 Pew Forum on Religion & Public Life look at "switching" found more than 10 percent of American adults became Nones after growing up within a religious group.
Chaves says there's another dimension to the unbelief trend worth watching.
"Americans famously say they believe in some variation of God. Over 90 percent do," Chaves says. "But it used to be 99 percent decades ago. The change is slow, but we can see it coming."