It might be more helpful in the long run to talk about a more neutral subject that involves believing something without sufficient evidence to justify the belief. If they believe in Bigfoot or ghosts, talk about that. You get to use all the same reasoning on a topic that isn't part of their core identity, and if you win that debate and they never change their mind about their religious belief, at least they don't buy into Nessie or whatever anymore.
I'm not anti-Christian. I'm anti-stupid.
