More from the study...

... Why it matters: "This is a broad-based social change," says Alan Cooperman, the director of religion research at the Pew Research Center.

- - - "We've had rising shares of people who don't identify with any religion " so-called 'nones' " and declining shares who identify as Christian, in all parts of the country, in all parts of the population, by ethnicity and race, among both men and women, and among people at all levels of the educational spectrum," he says about the survey findings.

By the numbers: Fewer than half of 18- to 29-year olds identify as Christian (45%), and nearly the same portion have no religious affiliation (44%), according to Pew's Religious Landscape Study, which surveyed more than 35,000 Americans.

- - - Meanwhile, 78% of those 65 and older identify as Christian.

Religiously unaffiliated adults came in at 29%, up from 16% in 2007, according to the study.