Elaine Pagels, the famous historian of early Christianity, once told a revealing story about the social world behind the scenes of high-powered biblical scholarship. As a young up-and-coming professor at the annual meeting of the Society of Biblical Literature, she was invited to a closed-door, after-hours smoker. The men there (Pagels was the only woman) were all prominent Bible scholars. Many of them didn't even believe in God, and those who still called themselves Christian were anything but orthodox.

The liquor flowed freely, and as these men got in their cups, they began to sing old gospel songs. To her astonishment, they knew all the tunes and words by heart. Then it dawned on her—these atheist and liberal Bible scholars must have grown up in evangelical churches.

Had Pagels herself grown up in evangelicalism she might not have been so surprised. Evangelicals have long known that it is easy for individuals and institutions—especially professors and universities—to slide down the slippery slope from orthodoxy to infidelity. Once down the slope, there's usually no climbing back up. It's a one-way street from evangelicalism to liberalism, a street that many individuals and colleges, and all the mainline Protestant denominations, have gone down. This bit of evangelical folk wisdom has a counterpart in sociology's sect-to-church theory. This theory claims that orthodox religious groups in a secular environment will gradually and inevitably become more like their environment. Once secularized, they do not again become orthodox.

As American evangelicals move into the 21st century, however, we may soon witness a new thing under the sun. Contrary to folk wisdom and traditional sociological theory, the mainline Protestant denominations ...