In 1972, a quarter of young adults in America -- 25 percent -- self-identified as Mainline Protestantism. In 2010, that was down to a mere 6 percent.
That's a 19 percent plummet, apparently.
This bit of data from LifeWays Research is consonant with two major stories that are regularly repeated about religion in America today: 1) that the mainline churches are in decline, and 2) that an increasing number of people are giving up on religion altogether, and now you have these "nones."
A second bit of data complicates both these story lines, though:
Of the quarter of self-identified Mainline Protestants in 1972, only 4 percent said they attended a Mainline Protestant church on a weekly basis. In 2010, that number had declined by half, so only 2 percent of self-described Mainline Protestants also said they attended church every week.
This means that in the '70s, lots of people would say they were Episcopalian, Presbyterian, Congregationalists, etc., but that identification did not connect in their minds to the cultural practice of church attendance. Church identification was not taken to mean weekly church attendance.
It's possible that the dramatic change we're currently seeing is only a change in that idea of what it means to say one is a Mainline Protestant.
That is itself a kind of secularism, as religion's dominance has declined at least in the sense that's it's now increasingly okay to say you don't have one, but this is a lot, lot less than is usually claimed for secularism, and a lot, lot less than is usually claimed for these sorts of statistics.