Is bland beautiful? Almost never, most of us would say. But when it comes to religion in a diverse society, the answer may be yes.

This is the chief, if probably unintended implication of American Grace, which I take to be the most successfully argued, comprehensive sociological study of American religion in more than half a century. Robert Putnam and David Campbell harvest a generation of research and mature reflection about how religious affiliations of all kinds divide and unite Americans of different generations, regions, sexes, educational levels, and ethno-racial groups. Will Herberg’s endlessly discussed Protestant-Catholic-Jew, a book of 1955, was not remotely as methodologically self-conscious and as empirically grounded as is American Grace, but one must go back to Herberg to find so striking a single volume purporting to explain the religion of an author’s contemporary Americans. If this coming generation of scholars and journalists allow Putnam and Campbell to define the terms of conversation to the extent that our predecessors allowed Herberg to perform this role, we will be in fine shape.

Why does this book prompt the suspicion that bland may be beautiful? Because Putnam and Campbell argue that the decline of intense, sectarian devotion to any particular faith enables religious believers to be more tolerant and appreciative of ideas and practices different from their own. Putnam and Campbell’s central, data-driven theme is the fluidity of American religion. Americans move in and out of religious affiliations with dizzying frequency. While in other societies religious identity is more often perceived “as a fixed characteristic,” they explain, in the United States “it seems perfectly natural” to refer to one’s religion as a mere “preference.”

All this mobility in an immigrant-receiving society with multiple ethno-religious groups creates, especially in recent years, high levels of religious diversity within families. One half of Americans today are married to someone who came from a religious tradition different from their own, and when you start counting cousins and in-laws you have extended families in which most people are intimately connected with several individuals from a variety of communities of faith. This reality leads Putnam and Campbell to their charming “Aunt Susan Principle.”

Just about everyone has an Aunt Susan, the kind of relative who is so saintly that you know she will get to heaven (if you believe there is such a place, but let’s put aside differences of opinion about that), even if she is an atheist or a Presbyterian or a Buddhist or something else that you are proud not to be. Familiarity and love conquer sectarianism and breed tolerance. The “My Friend Al Principle” encapsulates the same situation for non-family acquaintances. You greatly admire Al, your office co-worker. So, you have no doubt he’ll make it to heaven even though he happens to be a Jehovah’s Witness (horrors!) and you are an Episcopalian.

Putnam and Campbell well understand that American society is sharply polarized by religion, and that this polarization often parallels political polarization. They believe they have solved the paradox of how a religiously polarized society can also be a religiously tolerant society. The answer is that Americans do not get too deeply entrenched in any one, particular religious affiliation.

But some people do. “True believers” is American Grace’s term for those who are intensely religious, and as a result have little use for folks with beliefs different from their own. Putnam and Campbell insist that only about ten percent of Americans are true believers, but the true believers turn out, predictably, to be among the least tolerant of same-sex relationships, non-marital co-habitation, abortion, divorce, and of all kinds of pluralism. Even apart from these extremists, however, conservatism of this type is more prevalent within the most homogeneous and stable of religious groups, such as Mormons and evangelical Protestants, than among the most fluid, such as Jews, ecumenical Protestants, and agnostics. Here, American Grace is consistent with Robert Wuthnow’s findings concerning “exclusivist Christians” in America and the Challenges of Religious Diversity.

This demonstrable tension between intensity of belief and pluralistic tolerance is where the beauty of blandness becomes visible. Putnam and Campbell are not as forthright as they might be about the implications of their work. Clearly, they understand religion as a fine thing, providing needed networks of belonging and systems of meaning. Indeed, American Grace is a relentlessly generous book, filled with hope that the intolerance and sectarianism found among the “true believers” can be contained. The authors warn that the future is far from certain, and that the current association of religion with conservative politics might well be reversed. Religion in this book is, by and large, warm and wonderful. But their research leads to the conclusion that the warmest and most wonderful kinds of religion—and the kinds most compatible with a diverse, democratic society—are the kinds of religion that adherents regard as disposable, as something one is willing to trade away.

I hasten to acknowledge that American Grace offers an imposing and altogether welcome array of detailed information and wise reflection about countless aspects of religious life in the United States today. This very rich work’s value should not be reduced, as I risk doing here, to its most obvious and most general implication for the sociology of religion.

American Grace reminds me of one of the most striking findings in another recent sociological study, Christian Smith and Patricia Snell’s Souls in Transition: Religious and Spiritual Lives of Emerging Adults. Invoking H. Richard Niebuhr’s legendary put-down of liberal Protestantism’s drift away from doctrinal particularity, Smith and Snell remark that today’s younger Christian believers apparently feel no objection to “a God without wrath” who “brought men without sin into a kingdom without judgment through the ministrations of a Christ without a Cross.” Harry Emerson Fosdick “would be proud,” Smith and Snell allow mischievously, to listen in on the religious chatter of today’s young adults, including evangelicals whose grandparents hated Fosdick, because even if they’ve never heard of Fosdick they talk just like him.

Putnam and Campbell offer their own families as both normative and representative of life in our own time, which might be called “the era of Fosdick’s revenge.” Campbell is a Mormon with Protestant and Catholic ancestors. Putnam was raised a Methodist but converted to Judaism, while his sister married a Catholic and had three children all of whom are now evangelicals. Will all these Putnams and Campbells, like Aunt Susan and friend Al, get to heaven? Only if they remember the chief lesson of this book: don’t take your religion too seriously.