Julian Cope presents Head Heritage

Money and Power. The church could afford to pick the single tribe of christian gaels from Ireland who lived in Scotland from 500ad onwards, and back them in a bid to rule which ended with the all the other non gaelic cultures being subsumed to the point that people still forget today that brythonic was spoken here for a lot longer than the gaelic ever was.

As to magic, or miracles, Derren Brown would have been hailed as a magi or saint in those days. Druids expanded their memories from an early age, just as modern day magicians do, with hermetic memory techniques, for instance. It isn't a far stretch to see them applying it the same way, to bamboozle people.