You don't.
That's not all that unreasonable is it? They aren't asking you to behave any differently are they?
I mean, I'm a staunch atheist, but if you define god only as the creator of the universe, then you can have that god, because here we are, and here we weren't before.
Now, if they take it farther and start asking you do DO things because of this god.. You can punch all the holes you want in that stuff, and you won't need anybodies help with arguments either.