For several decades now, I have been hearing pollsters -- with organizations on the left, right and in the middle -- asking some very similar questions about trends on the left side of the marketplace of ideas that is American religion. In fact, they have often, been asking precisely the same question.

That doesn't happen very often. So when it does, I think that it behooves those of us who report and write about religion to pay attention.

Here's the key question: If more and more Americans are moving toward liberalism on questions of faith and morality, then why are the membership statistics continuing to spiral down in doctrinally liberal churches?

That question came up again this week in our "Crossroads" podcast, when host Todd Wilken and I talked about the heritage of research done by the late George Gallup, Jr., and how it related to a new LifeWay Research survey -- the "Theological Awareness Benchmark Study" conducted for Ligonier Ministries of Orlando, Fla. Click here to listen in.

Once again, we saw trends similar to those noted in the famous "Nones on the Rise" survey from the Pew Forum on Religion & Public Life. There is no question that the so-called "nones" -- truly secular, vaguely spiritual and religiously unaffiliated Americans -- are growing as a liberal segment of American religion. However, the doctrinally orthodox camp on the cultural right is hanging in there, at roughly 20 to 25 percent of those polled.

That reminded me of the following exchange I had with a progressive pollster about that Pew study in 2012:

This survey shows that "it's going to be much more difficult for mainline churches to turn things around simply by focusing on higher levels of commitment," said political scientist John C. Green of the University of Akron, after a briefing at the annual meeting of the Religion Newswriters Association of America. This research was a cooperative effort with the PBS program Religion & Ethics NewsWeekly.

Part of the problem is that fewer Americans remain committed to supporting religious institutions and a high percentage of those who do seem to favor faiths that embrace the very doctrines and traditions the unaffiliated often reject. It also appears that young people who are rejecting traditional faiths -- during the past five years in particular -- are quitting organized religion altogether, rather than joining progressive institutions.

"It's going to be hard for something like a 'fewer Methodists, but better Methodists' approach to work because these mainline churches are already so small and there are so many of them," said Green. "The mainliners will have to find their niche. But who are they? What do they believe? Do they know?"

LifeWay Research President Ed Stetzer told me something very similar when discussing his team's new study, which focused on specifically doctrinal questions (including my "tmatt trio" set). The bottom line is that very few doctrinally liberal people feel the need to attend and support an institutional church. Their faith is perfectly at home in pubs, gyms, shopping malls and movie theaters.

Here is a large chunk of material about that from my Universal column this week -- discussed in the podcast this week. Note what is happening in the "mushy middle" and how that relates to the trends on the left.

The spring survey is the latest to show that most Americans affirm traditional religious beliefs, sort of, but turn into "cafeteria" believers who pick and choose whatever makes them feel comfortable when it comes to doctrinal specifics, said LifeWay President Ed Stetzer. Things can get foggy and confusing in the "mushy middle" of the religious spectrum, where Americans worship a "Christian-ish god," rather than the God of traditional Christian faith. ...

The real action ... is among "nominal" believers. Stetzer described this "mushy middle" as the 25 percent of Americans who are "cultural Christians" in name only, and another 25 percent made up of "congregational Christians" who may visit sanctuary pews at Christmas and Easter.

The bottom line: Increasing numbers of ordinary Americans no longer think of themselves as "sinners," especially on issues of sex, and don't really care what their pastors or churches think about that. In fact, 82 percent say their church has no authority to "declare that I am not a Christian." On the Bible, 48 percent said it was the "Word of God," while 45 percent said the scriptures were written for each person to interpret as he or she sees fit.

Heaven is real for 67 percent of Americans and 61 percent believe in hell. Then again, 55 percent of "mainline" Protestants and 67 percent of Catholics, compared to 19 percent of evangelicals, believe there are many different ways to find salvation. Only 55 percent of "mainliners" and 45 percent of Catholics affirmed that salvation is found through Jesus alone.

In all, 67 percent of those polled affirmed that most people are basically good, even though everyone sins a little. Rather than stressing repentance and grace, most Americans see salvation as a "pull yourself up by your bootstraps" process, said Stetzer.

However, as noted by Green two years earlier, the nominals are sliding all the way over into the "nones" and not into the progressive churches that actually share their modernist takes on crucial theological and cultural issues.

That's the bad news for the institutional religious left. So should the institutions on the doctrinal right be celebrating? Not really. Let's end with this sobering observation:

"We can expect the movement by 'nominals' into secularism to continue," said Stetzer. "Would people in the mushy middle choose to identify with the right, which will increasingly be portrayed as harsh and judgmental and even bigoted? No way. More and more 'nominals' will join the 'nones.' "

Enjoy the podcast.