After decades of soaring growth, the phenomenon of Protestant megachurches — behemoths of belief where 2,000 to 20,000 or more people attend weekend worship — may be stalled.

Willow Creek Community Church in South Barrington, Ill., the granddaddy of "seeker-sensitive" megachurches geared to attract the spiritually curious, is on a mission to rev the engines.

On paper, megachurches look like a trend still on the rise. Their total number rose from 600 in 2000 to more than 1,250 in 2005, says sociologist Scott Thumma of the Hartford Institute for Religion Research in Hartford, Conn.

On Outreach magazine's 2008 list of the largest 100, even the smallest get more than 7,000 people attending. But some of the biggest, including Joel Osteen's Lakewood Church in Houston, with 43,500, showed slight declines.

Experts see more troubling concerns than slowing growth: No measurable inroads on overall church attendance and signs that many churchgoers are spectators, not driving toward a deeper faith.

"You can create a church that's big, but is still not transforming people. Without transformation, the Christian message is not advanced," says Ed Stetzer, head of Lifeway Research in Nashville, which did the Outreach study.

The unchurched remain untouched.

While the number of people who say they attend at least once a week hovers around 30 percent year after year, the number who say they "never" go to church climbs.

The tally of "nevers" varies from 16 percent in Gallup surveys to 22 percent in the General Social Survey, conducted by the National Opinion Research Center at the University of Chicago, and 32 percent in an Ellison Research survey this year.

The new "nevers" come from the pool of people who once attended monthly or a few times a year.

Many slide away from church to find other answers to their spiritual quest or find another church where the preaching or music or family programs better suit their style.

"The megachurch story is not really about growth, it's about shifting allegiances. People want to feel good about who they already are," says Philip Goff, director of the Center for the Study of Religion and American Culture at Indiana University in Indianapolis. "If church is too challenging or not entertaining, they'll move on."

Many who do stick may be "spiritually stuck," as well, according to a spiritual inventory conducted by Willow Creek.

The study, now being marketed to churches nationwide as a self-assessment tool, found that many who attend church are not progressing from beginner believers to become "fully centered in Christ" - deep in Bible study, prayer and service.

In response, Willow Creek founder and senior pastor Bill Hybels has changed his sermons to more directly challenge worshipers at every level. Willow Creek has launched dozens of Wednesday mini-classes focusing on spiritual growth.

Willow Creek is still "seeker-obsessed," says Hybels. "But today's seekers are different" than years ago.

Today, he says, "I don't think anyone is wandering around looking for a mild dose of God. They want to know: 'What would a life centered on Christ look like in my life? What would that feel like? How do I go about it?' "