Shankar Vendantam has been reviewing the literature on how church attendance numbers are derived, and he has some relevant commentary.
First, there's the old do-you-believe-in-God style of research, which gets you the results of an opinion poll, but is not to be confused with pew-sitting. Beyond that, though, apparently you can either ask a person straight-up if they went to church on Sunday, or you could say, "Tell me what you did Sunday and for how long you did it and with whom," and allegedly you get a more honest answer.
The better studies ascertain whether people attend church, not what they feel in their hearts. It's possible that many Americans are deeply religious but don't attend church (even as they claim they do). But if the data raise serious questions about self-reported church attendance, they ought to raise red flags about all aspects of self-reported religiosity. Besides, self-reported church attendance has been held up as proof that America has somehow resisted the secularizing trends that have swept other industrialized nations. What if those numbers are spectacularly wrong?
"If Americans are going to church at the rate they report, the churches would be full on Sunday mornings and denominations would be growing," wrote C. Kirk Hadaway, now director of research at the Episcopal Church. (Hadaway's research has included evangelical congregations, which reported sharp growth in recent decades.)
In a more recent study, Hadaway estimated that if the number of Americans who told Gallup pollsters that they attended church in the last week were accurate, about 118 million Americans would be at houses of worship each week. By calculating the number of congregations (including non-Christian congregations) and their average attendance, Hadaway estimated that in reality about 21 percent of Americans attended religious services weekly—exactly half the number who told pollsters they did.
I don't think religious intensity necessarily explains how religiosity becomes part of one's identity. Canada and the United States are quite different today in terms of their religious intensity and the importance they attach to the role of religion in public life, yet citizens in both countries greatly exaggerate their church attendance.
Vendantam offers some thoughts about why this is so, but seems equally interested in pointing out the simple fact of the incongruity between what we say we do and what we actually do. That seems valid enough, but assuming it's an argument with traction, where would it take us?
It strikes me that one place to go next is to an internal gut-check: Perhaps you could ask people who report church attendance what value they derive from the experience. Do honesty or truth-telling appear anywhere on the list of reported responses?