I don't think you need to go to war so much as just ask leading questions. We have four very Christian grandparents who send religious stuff. We've let the kids have it, but discussed it. They are 15 and 17 now and both atheist. There wasn't any need to call their grandparents stupid or suggest that that their grandparents didn't love them. A little critical thinking goes a long way.
If there is a god, I want to believe that there is a god. If there is not a god, I want to believe that there is no god.