I feel for you. Its "christians" like that who make other Christians look bad.
Just tell him he's possessed by God. And that it's a bad thing.
If he wants to waste his life away letting something silly contol his life, let him. Let him have no fun if he wants. Let him be afraid to make up his own mind, if he chooses. And there is no devil, it's just a scare-tactic made up to make people behave.
Don't blame God or religion. It sounds like your dad is either afraid of his own mortality (maybe he's done some things he regrets) or he's getting senile (more likely given he's worried about the cable being Satan's work). Have your mom take 50% of their assets and put it in a separate account to protect her financial future.