The United States is a nation that was both founded by religious people and built around a guarantee of liberty from state-imposed religion. The religious landscape has changed substantially over the past three centuries, with the addition of faiths in America that range from Islam and Judaism to the Church of Jesus Christ of Latter-day Saints; today's landscape of worship includes megachurches with huge congregations, storefront tabernacles, spiritual advisers.
And mostly, Americans accept that diversity with remarkable tolerance. What kind of force is religion in American life today?
A new book by professors Robert Putnam and David Campbell seeks to examine some of the changes. It's called American Grace: How Religion Divides and Unites Us.
One of the biggest changes over the past 20 years has been that more and more Americans, when polled, cite no religious affiliation at all. That group, which Putnam and Campbell call the "the nones," "has been skyrocketing actually in the last 15, 20 years," Putnam says.
"So it's now, roughly speaking, 35 percent [to] 40 percent of younger Americans ... who say that they have no religious affiliation."
That's a big change. For many years, the researchers say, only about 5 to 7 percent of Americans felt they belonged to no religion. The shift, Putnam says, is "a quite novel and interesting, significant development."
As for the Americans who do belong to religious groups, tolerance is flourishing among them, too.
The Aunt Susan Effect
One huge factor in that increased tolerance? More marriages across once-rigid religious lines.
"By now more than half of all Americans are married to someone in a different religious or faith tradition," Putnam says. "Our friendships increasingly cross religious boundaries." And as Putnam says, "it's hard to demonize people of a certain religion when you have someone like that in your own family.
"We call that the 'Aunt Susan effect,' " he says. "Almost every American has an 'Aunt Susan' because of intermarriage and so on."
No matter what religion this putative Aunt Susan follows, knowing and loving her makes it hard to judge her.
"You know that your faith says ... she's not going to go to heaven, but I mean, come on," Putnam says. "[It's] Aunt Susan, you know, and if anybody's going to heaven it's Aunt Susan. So every American is sort of caught in this dilemma, that their theology tells them one thing, but their personal life experience tells them to be more tolerant."
The God Gap In Voting
Another shift in the American religious spectrum is how religion correlates to political beliefs. Or, more precisely, your degree of religiosity.
"How frequently you attend church, or interestingly, how frequently you say Grace ... measures like that, how religious you are, have a pretty strong correlation with how you vote and which party you prefer," Campbell says.
Not too long ago, such a link between religiosity and political belief was almost unheard of.
"At the election of Dwight Eisenhower," says Campbell, "there was literally no God gap in voting. Eisenhower was just as likely to pick up votes from people who attended church frequently as those that did not attend church frequently."
Unsurprisingly, American Grace points to a current correlation of more religious people with the Republican Party, though Putnam points out that African-Americans are an important exception to the rule. They are the most religious group in America, but are more connected to the Democratic Party.
"We've reached a point where there really is a pretty stark political divide on the basis of, not so much the particular denomination you attend, but rather the intensity of your religious commitment," Campbell says.
What about the groups that are both deeply religious and more connected to the Democratic Party? People often end up adjusting their religion to meet their political ideas, or vice versa.
One big reason for that growth among the nones is precisely such an adjustment.
"There's been a kind of a quiet backlash among young people against this politicization of religion," says Putnam. And as a result, young liberal-minded people have often ended up avoiding religion because they associate it with conservative Republicanism.
And yet the title includes the notion of American unity, which the authors say is a big focus of the book.
"The 'American Grace' of our title is meant to refer to the unity that we find along religious lines," Campbell says. "We find people actually coming together [from] different faiths and different religions."
The America of today, Campbell says, has "managed to accomplish something that is historically and internationally unique — that we're a religiously devout country, a religiously diverse country and a pretty religiously tolerant country."