Americans who have been paying attention to arguments about religion in recent years know that two statistics have caused the most chatter in the digital public square, as well as coverage in the old-school press.

First, there was the omnipresent statistic from 2016 polls indicating that 81% of white evangelicals backed Donald Trump. Meanwhile, the thrice-married New York billionaire captured 60% of white Catholic votes and – here's the shocker – 58% of votes by "Protestant-other" Christians, a niche dominated by America's more progressive mainline churches.

But that wasn't the hottest of the hot statistics, the one that caused a tsunami of debates about the future of American religion. That honor would have to go to the Pew Research Center's reports on the soaring number of people who are "religiously unaffiliated," or "Nones." In 2012, Pew noted that "Nones" had quickly risen to be one-fifth of the U.S. public.

Earlier this year, political scientist Ryan P. Burge of Eastern Illinois University dug into the 2018 General Social Survey, crunched some data and then took to Twitter to note that Americans with ties to no particular religious tradition were now about 23% of the population. That percentage is slightly higher than evangelical Protestantism and almost exactly the same as Roman Catholicism.

"At that point my phone went crazy and I started hearing from everyone" in the mainstream media, said Burge, who is co-founder of the Religion In Public weblog. "All of a sudden it was time to talk about the 'Nones' all over again."

Burge recently started another hot discussion on Twitter with some GSS statistics showing trends among believers – young and old – in several crucial flocks. For example, "19.9% of young people are evangelical, compared to 22.9% of those over 65." At the same time, "21% of young people are Catholic. Compared to 24.3% of those 65+."

Those stats show some decline, but not collapse. Then again, Burge noted that, "4.5% of those under 35 are mainline Prot. vs. 20.6% of those 65+."

This "mainline" label has long been used by social scientists to refer to the denominations known as the "Seven Sisters." In descending order by size, they are the United Methodist Church, the Evangelical Lutheran Church in America, the Presbyterian Church (U.S.A.), the Episcopal Church, the American Baptist Churches USA, the United Church of Christ and the Christian Church (Disciples of Christ).

"Mainline is a tradition that has declined rapidly over the last 40 years, while nones have risen just as dramatically," tweeted Burge. "It seems that a lot of kids who were raised mainline left religion entirely – their parents stayed. But, that means mainlines are going to face an age cliff soon."

With that in mind, it's hard not to connect the aging mainline numbers with the rising tide of the "religiously unaffiliated." In the 2018 GSS, Burge tweeted, "33.8% of those under 35 are nones, compared to 11.5% of those over 65."

This raises a question that has baffled researchers, he noted. If America is moving left on many moral issues, why have progressive churches declined so sharply, losing between 30-50 percent of their members since the 1960s?

"That's the question of the year and that's been true for many years now," said Burge, who was raised Southern Baptist, but is now active in the mainline American Baptist Church USA.

"People are either being intentionally and intensely religious – like really active evangelicals, or conservative Catholics, or yada yada – or they're leaving the more moderate churches that we've always seen as holding down the middle of religious life in this country," he said. "We've ended up with two trends that appear to be related. We're losing moderation in the political space in American life and we're losing moderation in the religious space, as well."

The political implications of these trends are complex, he said. But there does seem to be a Big Idea at the heart of it all: Millions of Americans, especially young adults, have decided that "religious" equals "Republican."

Burge put it this way on Twitter, describing how doctrines can affect public life: "There is no religious group in the U.S. that places itself halfway between the political parties. Instead, every religious tradition has a 'baked-in' political preference now. Nones favor Democrats. Christians (by and large) favor the GOP."

Terry Mattingly writes this weekly "On Religion" column for the Universal syndicate. Reprinted with permission of the author.