Between 2007 and 2014, the number of adults who self-identified as Christian fell from 78.4 percent to 70.6 percent, or 172.8 million people, driven mainly by drops among Roman Catholics and some Protestants, according to the Pew Research Center poll.
Non-religious U.S. adults who identify as atheist, agnostic or of no particular faith have seen their ranks swell over that same time period by about 6 percentage points to 22.8 percent of the population, or 56 million people.

"One of the most important factors in the declining share of Christians and the growth of the 'nones' is generational replacement," the report said, referring to respondents who chose no religious identity. "As the Millennial generation enters adulthood, its members display much lower levels of religious affiliation."

The report, which compares to a similar Pew survey conducted in 2007, found that while the decline in Christianity was most pronounced among younger adults, there were significant declines across ages, races, genders and geographic regions.