In "American Religion: Contemporary Trends," author Mark Chaves argues that over the last generation or so, religious belief in the U.S. has experienced a "softening" that effects everything from whether people go to worship services regularly to whom they marry. Far more people are willing to say they don't belong to any religious tradition today than in the past, and signs of religious vitality may be camouflaging stagnation or decline.

Today, as many as 20 percent of all Americans say they don't belong to any religious group, Chaves found, compared with around 3 percent in the 1950s. Yet, those people aren't necessarily atheists, agnostics or others. Instead, about 92 percent of Americans still profess belief in God, they just don't use religion as part of their identity.

Even signs of robust religious faith may not be what they appear, Chaves found. The strength of religious conservatives in politics, for example, has coincided with a growing disillusionment about faith's role in the public square. Chaves found that between 1991 and 2008, the percentage of Americans who strongly agreed that religious leaders should stay out of politics rose from 20 percent to 44 percent.

Anderson thinks the change is better described as a shift than a decline, as people become more willing to leave the denominations or faiths in which they were raised and look elsewhere for spiritual nourishment.