Partly what fbhjr said, and partly that, simply by being an atheist in public, you are [a] denying the very existence of the god they worship and [b] implying that they are an idiot for believing in an imaginary being. No matter that a person's opinions about this are his or her own. (I'm very fond of the notion that, at core, every religion has one member.)

One very interesting way of attacking atheism that I've noticed is for the non-atheist to say the atheist is actually an agnostic, because if sufficient proof in the existence of a God was found, said atheist/agnostic would believe. They don't seem to grasp that, by the lights of an atheist, such proof would be so profound that, yeah, confronted with it, pretty much anybody would believe.

However, the kind of proof we're likely to have, i.e., the same "proof" we've had all along, makes the existence of God -- certainly one as they envision -- so far out of the realm of possibility that it may as well be impossible.