OMG...
My MIL, who is THE holy roller, a born again charismatic Christian, invites them in to debate.
Serves 'em right, I think!
Me, I am polite, tell 'em God Bless, but I am very happy in my own church (which I haven't been to in a couple of years now, which is why I am probably still very happy with it!).
edit: My great uncle, who was a crazy, scary old man, told them that he worshipped Satan. That seemed to do the trick....