Older Adults: People over age 50 are more likely to attend church, but seniors are also more resistant to the gospel. Researcher Thom Rainer categorized unchurched people in five groups. Twice as many over-50s were classified "most resistant," compared to the average of all unchurched persons. Rainer said younger people were much more likely to have interest in spiritual things.

Their church attendance, however, is another matter. Of those in their 20s, 31 percent say they attend weekly, compared to 53 percent of those over age 60.

20-Somethings: AWOL. Pollster George Barna says younger people drop out of church because they struggle to find a place. They are one-third less likely than older adults to attend, give financially, or read the Bible. Barna is most concerned about the 58 percent decline in attendance among 18- to 29-year-olds who were faithful churchgoers in their teens. He blames their quest for personal fulfillment and the tendency of churches to overlook young leaders. Only 4 percent of the 20-somethings report serving in church leadership.

Smaller Churches: The smaller church is the right setting for many adults under age 35. Boomers like larger churches—one-fourth attend churches of 500 or more, compared to one-sixth of busters. Barna credits the disinterest of busters in boomer-led organizations, and their desire for strong community. While some have predicted their demise, Barna says small congregations will remain prevalent. About 60 percent of U.S. churches have 100 or less in attendance each weekend.

—with information from churchcentral.com, barna.org, and USA Today.