GWEN IFILL: Now to surprising new findings about our changing religious landscape, and how and if we believe.

Jeffrey Brown has our conversation.

JEFFREY BROWN: The U.S. remains an overwhelmingly Christian country. That hasn't changed, but a new survey shows a significant drop in the number of Americans who identify as Christian.

The survey was done by the Pew Research Center. It showed that, in 2007, 78 percent of Americans identified as Christian. By last year, the percentage had dropped to under 71 percent. Those years have seen a dramatic rise in the number of Americans who say they are religiously unaffiliated, from 16 to nearly 23 percent.

The largest drop was in mainline Protestant denominations, but the number of Catholics also fell. Several non-Christian religions, Islam and Hinduism, saw modest gains.

Alan Cooperman is here. She's the director of religious research at Pew. Also with us is Reverend Serene Jones, president of the Union Theological Seminary in New York City.

And welcome to both of you.

And, Alan Cooperman, let me start with you.

One aspect of this that might surprise people is just how widespread this drop is. Did that strike you?

ALAN COOPERMAN, Pew Research Center: Absolutely, Jeff.

I mean, I think the important thing for people the realize is, this is really widespread, broad-based social change. It's taking place not just in the big cities or in the Northeast. It's taking place in every region of the country, including in the Bible Belt, among men and women, among blacks, Latinos and whites, among older people and younger people, and among people with college degrees and those with only high school degrees.

JEFFREY BROWN: And, Dr. Jones, what — does it jibe with what you see happening around you? Are you surprised at all?

REV. SERENE JONES, President, Union Theological Seminary: Yes, it's surprising to see the statistics lay it out so clearly, but, on another level, it's not surprising at all. It's exactly what we all look around when what we see in New York City or — I'm from Oklahoma — when I walk through the fields of the small town I grew up in. It's the reality of the U.S. we live in today.

JEFFREY BROWN: Tell me a little bit more. What do you think explains it?

REV. SERENE JONES: Well, it's very interesting.

At Union Theological Seminary, we're seeing the unaffiliated, this now growing group, fastest growing group, actually walking through the doors of seminary. So I have occasion quite often to talk to them about why they have left religious communities and what it is about spirituality in general that seems to still attract them.

And I think one of the reasons why we're seeing the bigger drop in the mainline than we are in the evangelical, for instance, is that, in a lot of mainline communities, the line between what it means to be in the church and out of the church is a very fluid line. And we see the unaffiliated, you know, perhaps coming back to church once or twice a year, not quite sure what they think about religion, whereas, in evangelical communities, that demarcation is much sharper and the pressure not to leave the community is greater.

I think it's interesting in a mainline context to ask, what are the incentives to stay? And I think it's causing the mainline churches to do a lot of soul-searching, first of all, to figure out why people are leaving, but secondly to ask this really interesting question: Is it always bad that churches shrink? And those are sort of interesting political and theological questions that the survey doesn't touch upon, but I think we're being prompted to deal with.

JEFFREY BROWN: Let me ask Alan Cooperman about what the survey does touch on, the unaffiliated, this rise, because it too is fairly widespread, right? We have a graphic of age demographics. Tell us about that.

ALAN COOPERMAN: Well, it's especially concentrated among younger generations.

So, while it's true that it's taking place amongst older adults, as well as among younger adults, it's really startling. Among the youngest millennials, 36 percent are unaffiliated, whereas, in the country as a whole, it's down 23 percent. Among older generations, it's a much smaller percentage.

And so a lot of what's happening, Jeff, is what we call generational replacement. It's a nice way of saying that the older generations, which were very heavily Christian, are passing away, and they're being replaced by younger cohorts that are far more unaffiliated, not only than older generations are today, but than those older generations ever were.

JEFFREY BROWN: And let me stay with you, because this is another question that follows. Is this an affiliation or denominational-type issue, or is it an issue of actual religious faith? How much do we know about that distinction?

ALAN COOPERMAN: Oh, these labels matter.

Now, of course, within every religious group, and within every group of people who identify themselves as Christians, there is a spectrum. There are people who are — who don't believe and there are people who are very strong believers. There are people who attend church regularly and people who seldom or never go. Within every group, there is a spectrum.

But, for sure, Jeff, those who are unaffiliated are much less likely to attend religious services. They pray less often. Religion is less important to them. And there are also political concomitants. For example, the unaffiliated as a whole lean almost as strongly Democratic as white evangelical Protestants lean Republican.

JEFFREY BROWN: Well, Serene Jones, you asked that question about how much this matters to particular churches and particular denominations. What do you — can you begin an answer here for us?

REV. SERENE JONES: Well, it's a challenge to all the denominations, the mainline in particular that I'm most familiar with at Union, to ask why this drift is happening.

But I think one of the biggest challenges when asking that question is to say, look, religion is not like going to — you know, it's not a campaign. It's not a sport event. It's not about numbers. It is, though, interesting to look at demographics and say, what do they tell us about the world in which these religious communities find themselves?

And one of the interesting things I'm seeing at Union — and the demographics in the new study bear this out — is the religiously unaffiliated are growing in numbers, but we're also seeing an uptake in the numbers of Pentecostal and charismatic black and brown Christians coming to seminary.

So at the two ends of the spectrum, we see growth, where in the middle we're seeing a flattening and at times a decline. So what does it mean that, at both of these that would appear to be different ends of the religious spectrum with expression — with relationship to the way they express their faith, are both sort of taking ahold of this younger generation?

JEFFREY BROWN: You know, Alan Cooperman, I want to ask you about a separate Pew study I saw slightly earlier, but it suggests that this U.S. path differs from the rest of the world. And other surveys suggested that the world is becoming more religiously affiliated.

ALAN COOPERMAN: Yes, the United States is on a path that in some ways looks similar to what's happened in Western Europe, what's happened in other parts of the developing world, where secularization has been taking place, where the unaffiliated are growing.

But you're absolutely right. The fastest growing parts of the world, in terms of population, are places like sub-Saharan Africa, South Asia, and these are places where almost everybody identifies with a faith. And so they are somewhat contrary trends. On the one hand, the world population as a whole is over time becoming, if anything, more religiously affiliated, while Western Europe and the United States are becoming less.

And that does raise very interesting questions. Will we be able to understand each other in the future?

JEFFREY BROWN: And, very briefly, do you expect what you see happening, the trends here in the U.S., to continue?

ALAN COOPERMAN: Well, on demographic grounds, yes.

Of course, there's a great deal that I don't pretend to be able to predict, but when we see the younger generations increasingly unaffiliated and even the younger millennials more unaffiliated than older millennials, I see no reason to think that this, at least in the immediate future, going to turn around. Fifty years from now, 100 years from now, I can't possibly know.

JEFFREY BROWN: We will talk then.

JEFFREY BROWN: Alan Cooperman and Dr. Serene Jones, thank you both very much.

ALAN COOPERMAN: Thank you, Jeff.

REV. SERENE JONES: Thank you.