Grandma, eat another sugar cookie and shut up. 3 of 4 Americans identify as Christian. Whether that are "really" Christian is not in your providence to decide. You made a weak-headed comment by somebody who should know better.
They may say they're christians but they are not. I can say I'm a supermodel, does my saying it make it so?
If you want to be a christian, fine, be one. But f you aren't a real one then you aren't one at all.
And there's probably some people who say they aren't, but are.
Can't use that argument to benefit one side of an equation.