Though giving to religion in the U.S. topped $100 billion in 2010—putting it at nearly 1 percent of GDP—no monthly index charts the nation's leading religious indicators. Even the Census Bureau, the nation's most comprehensive data-gathering enterprise, avoids collecting basic information about religion. No constitutional provision bars the collection of such information, but the government has long hesitated to intrude in the affairs of religious bodies. So most of what we know about religion in America is based on what individuals and groups reveal voluntarily to independent researchers. Fortunately, a small number of public foundations, most notably Lilly Endowment, the John M. Templeton Foundation and the Pew Charitable Trusts, continue to fund basic research on the state of American religion.

One of the most comprehensive recent surveys of American religious participation was reported in 2008 by the Pew Forum on Religion & Public Life. The survey included over 35,000 respondents, making it possible to present detailed demographic and attitudinal data on large and small religious communities.

The Pew report notes that the American religious landscape is increasingly fluid and competitive. Fully 28 percent of Americans have changed their religious affiliation since childhood, a percentage that rises to 44 percent when interdenominational "switchers" within Protestantism are included. The ranks of the religiously unaffiliated are now 16 percent of American adults and fully a quarter of those age 18 to 29.