I heard a wonderful lecture yesterday by Nancy Hardesty entitled: "Baptists, Methodists, Snakes and Fire: 19th Century Religion in the South." She started her story with 19th century Methodists who emphasized the idea of sanctification. Her argument was that once you start validating religious extremes, people always want to go further.
She traced the story from the holiness movement to faith healing to baptism by fire to speaking in tongues to handling snakes to drinking poison. Those who handled serpents argued that their practice was required by Mark 16:17-18, but drinking poison was optional because the verse says: "if they drink any deadly thing it will not hurt them..." Snake handling is illegal in every state except West Virginia, where member of churches that followed the practice had enough political clout to prevent the passage of such a law.