@Terry: I agree with what you've said here, in the abstract. However....
When a theist goes so far as to describe their deity in great detail, outlining his supposed qualities, powers, etc, then at that point, one is able to take the implications that such a deity would have on the world, and contrast and compare this to the world our senses present to us. When we find blatant, irreconcilable differences in these two universes--the one our senses indicates and the one this god would imply--we are able safely to say "This version of god does not exist, and in regards to it, I am an atheist." All the while realizing, of course, that this is an "opinion", but based on positive evidence of how the perceivable universe exists in utter contrast to this god's supposed qualities and powers, and not based merely on lack of direct evidence of the god's existence.