Today pentecostalism claims nearly 500 million followers worldwide. An early stronghold was the American South, where believers spoke in unknown tongues, worshipped in free-form churches, and broke down social barriers that had long divided traditional Protestants. Thriving denominations made their headquarters in the region and gathered white and black converts from the Texas plains to the Carolina low country.

Pentecostalism was, in fact, a religious import. It came to the South following the post-Civil War holiness revival, a northern-born crusade that emphasized sinlessness and...