Scholars and observers who attempt to determine whether America is a religious nation are confronted with the challenge of having no definitive measure of “religiosity.” Some commonly cited points of evidence, such as opinion surveys and rates of religious service attendance, may be less revealing than they first seem.

As measured by responses to public opinion polls, the importance of religion in America is at first glance hard to deny. According to Gallup Organization surveys, 95 percent of Americans believe in God—a number that has remained remarkably consistent over the decades and that is much higher than in other industrialized countries (comparable polls show 76 percent of people in Britain professing belief in God, and 52 percent in Sweden). Yet for some, such poll results do not constitute an adequate gauge of religious faith of individuals or what religion means to...