There was a lot of believing that the end of the world was imminent (which I suppose is fairly understandable given what the current events of the 1640s must have been like to live through), making apocalyptic prophecies and so on. Then there's the Ranter belief that the light of Christ within every person was so important that true believers could never sin, and if they committed theft, adultery etc. it was actually perfectly all right with God—from a modern perspective this one seems like kind of a recognition of the fact that contemporary definitions of 'sin' often had little to do with actual wrong, but also kind of obviously full of terrible implications.

Pursuit of the Millennium also sounds interesting! This book had a little bit about older radical movements like the Lollards and the Familists, making the point that the radicals of the 1640s didn't spring into being out of nowhere, and radical medievals seem like a fascinating topic as well.