For the first time in U.S. history, Protestants do not comprise a majority of the population—but not because another religion is about to surpass it. Instead, the fastest-growing religious affiliation in America is, in fact, no religious affiliation.

According to a Pew Forum on Religion & Public Life survey of 17,000 adults, 48 percent say they are Protestants. Twenty-two percent answered Catholic, and 6 percent said they were of another faith, which includes Judaism, Mormonism, Buddhism, Islam and others.

But the really big move was the 20 percent who said they have "no religious affiliation," up from 15 percent who answered that way five years ago.

Within the religiously unaffiliated, about 20 percent answered they were atheist or agnostic, whereas the rest classified their religion as "nothing in particular."