The unaffiliated have had a much more dramatic trajectory, starting at just 1 in 20 of GSS respondents back in 1972. The nones experienced big jumps in 1993 and 1998, and have added, on average, 1-1.5 percentage points every 2 years for the last 10 years.
Their continued growth has been well-documented and at this point, expected. The big finding in 2018 is that those of “no religion” are now as common as evangelicals in the US, both about 23 percent of the population.
The difference between these groups is not statistically at this point; but I see no reason to think that the nones will level off anytime soon. If the nones maintain their growth while evangelicals stagnate, it is statistically inevitable that those of no religious faith will be the largest group in America in the next five years.
Mainline Protestants include those who identify as United Methodists, Episcopalians, or American Baptists. They have suffered tremendous declines over the last 40 years, going from 30 percent of the population in 1972 to just 10 percent in 2016. But in 2018, that share bumped to 11.8 percent.
It would unwise to extrapolate too much from a single-year increase. However, it may be the result of some moderate Protestants leaving evangelicalism for mainline denominations over Donald Trump’s presidency. Evangelicals declined 1.4 percent between 2016 and 2018, while mainline Protestants saw a slight increase. It is too early to say that these evangelical losses were mainline gains; however, if this trend continues or possibly accelerates in the next two years, it would give some credence to the claim that Trump has led to evangelical defections.
The GSS also shows that mainline Protestants have continued to move toward the left of the political spectrum, while evangelicals maintain their shift to the right. This continuing polarization may lead more moderate evangelicals to seek out a church that shares their political worldview. (Other surveys conducted after Trump’s election found that evangelical or born-again Christians who voted for Hillary Clinton were more likely to drop the label or see church attendance slide between 2011 and 2017.)