Another preacher question:
One of his sermons Cancels a Darkness card, or Cancels a Growing Dread card.
I think it's called "Intervention", right?
In FFP's other games, "cancel" is both a counterspell ability AND a destroy ability, to use M:tG terms. So, I'd say you can cast the sermon immediately when a new Darkness or Dread card is drawn before the card causes any effect. And, you can also use the sermon (every turn if need be) to try to remove an existing card that "Remains in Play".