I don't believe necromancy falls under druidism. More like under voodoo.

Also the concept of witches under the guise of church people, though largely exaggerated, is not that far fetched. The Church did use pagan hollidays as base for it's own to better supplant the old practices. Christmas and Halloween being the two largest and internationally recognizable that I can offer as an example. I'm not sure as well about the larger scale of penetration though. A single Saint and profit here and there, maybe. Let's not forget Pope Joan, the female Pope. She certainly managed to penetrate way over to the top.