Since the day this blog opened for business, your GetReligionistas been saying the same thing over and over about the structure of modern American religion.

Our sermon has sounded something like this. On one side of the American religion scene is a camp of religious traditionalists, of various kinds, that makes up about 20 percent of the American population. On the other side is a corresponding camp of roughly the same size made up of three groups -- true secularists, "spiritual" but not religious people and faithful religious liberals. In the middle is the post-denominational, non-doctrinal marketplace that I have, for the past decade or so, started calling "OprahAmerica."

There are all kinds of variations on this theme out there in the marketplace of ideas, from the work of sociologist James Davison Hunter, creator of the much-misquoted "Culture Wars" thesis, to research conducted by evangelical (whatever that word means) pollster George Barna, to the statistical portrait of the "anti-fundamentalist voter" painted by political scientists Gerald De Maio and Louis Bolce of the City University in New York. All kinds of people are talking about the "post-denominational age" in which religious labels have little meaning and, to be blunt about it, most of them have the facts on their side.

But, most of all, you can see the changing religion landscape in one article that your GetReligionistas keep begging readers to check out -- which is that "Tribal Relations" piece that ran in the Atlantic Monthly a few years ago, written by scholar John C. Green of the Pew Forum on Religion & Public Life and journalist Steven Waldman, the Beliefnet.com czar. The key is that there is a true left and a true right in American religion and culture, but that these opposing doctrinal camps are not where the real action is these days. For better and for worse, the heartbeat of America is found in the mushy middle -- even on a topic like abortion.

Now we have another study that shows just how confusing things are getting inside and outside all of those pews and pulpits. Here's the top of the USA Today report by veteran Cathy Lynn Grossman, which focuses on the modest growth of honest unbelievers and the vaguely spiritual believers:

When it comes to religion, the USA is now land of the freelancers.

The percentage. of people who call themselves in some way Christian has dropped more than 11% in a generation. The faithful have scattered out of their traditional bases: The Bible Belt is less Baptist. The Rust Belt is less Catholic. And everywhere, more people are exploring spiritual frontiers -- or falling off the faith map completely.

These dramatic shifts in just 18 years are detailed in the new American Religious Identification Survey (ARIS), to be released today. It finds that, despite growth and immigration that has added nearly 50 million adults to the U.S. population, almost all religious denominations have lost ground since the first ARIS survey in 1990.

And here's the statistic that will grab the most headlines:

So many Americans claim no religion at all (15%, up from 8% in 1990), that this category now outranks every other major U.S. religious group except Catholics and Baptists. In a nation that has long been mostly Christian, "the challenge to Christianity ... does not come from other religions but from a rejection of all forms of organized religion," the report concludes.

If you want to explore some of these numbers yourself, click here for some of USA Today's nifty online graphics or, even better, chase this connection to the ReligionLink site for a collection of URLs for the study materials themselves and some additional commentary.

There are all kinds of subplots in this story -- from the mini-rise of the Wiccans to the ongoing decline of the oldline Protestants, from the changes linked to social mobility to the sad plateau occupied by many evangelical churches. But the overarching storyline is not new. We live in the age of smaller and smaller niches and fading brand loyalty. This has been the broad trend for several decades and now we have another study underlining that reality. Cheers.

But it is possible that the unbelievers are going to be "hot," in media terms, especially since -- again here's the news -- this adds to an interesting coalition on the cultural left that is climbing toward statistical parity with those dreaded "values voters" of old. Then again, house-church evangelists from China are preaching on street corners in Berkeley. Things keep changing.

But the big, big trend remains the same. Welcome to the post-denominational age. You can see this in a nice summary passage in the Washington Post coverage:

The increase in people labeling themselves in more generic Christian terms corresponds strongly with the decline in people identifying themselves as Protestant, the survey found. People calling themselves mainline Protestants, including Methodists and Lutherans, have dropped to 13 percent of the population, down from 19 percent in 1990. The number of people who describe themselves as generically "Protestant" went from approximately 17 million in 1990 to 5 million.

Meanwhile, the number of people who use nondenominational terms has gone from 194,000 in 1990 to more than 8 million. ... The survey substantiated several general trends already identified by sociologists: the slipping importance of denomination in America, the growing number of people who say they have "no" religion and the increase in religious minorities including Muslims, Mormons and such movements as Wicca and paganism.

In other words, American religion is getting even more complex and nuanced. This means it is even more important that newsrooms contain Godbeat (or godsbeat) professionals offering a blend of experience, training and talent. That's obvious, but in the current business climate that is not going to be easy. But you knew that already, didn't you? After all, you're reading this weblog.