American mainstream Protestant churches must find a way to call back into the fold the baby boomers who have left the church and don't seem eager to come back, a prominent minister says.

"There is a group of people in America described as believers but not belongers," said the Rev. John Mulder, president of the Louisville Presbyterian Theological Seminary in Louisville, Ky.

A national survey showed that 6 million Americans said they were members of the Presbyterian church, but the church rolls have only 3 million members.

"They are people who don't go to worship, are not members of the church, but who in their own minds identify themselves as Presbyterians." Mulder and two other researchers, all church historians, are part of a national study of U.S. Protestant churches sponsored by the Lilly Endowment of Indianapolis, Ind. As part of the national study, Mulder and his colleagues are researching the Presbyterian church membership decline as it reflects national trends.

"I've seen a couple of their extended interviews, and one of the prevailing reasons I've seen (for their leaving church) is that they're just capable of living their lives without reference to God. Or they say they are," Mulder says.