Carl, the problem is that "God" is a word that has no clearly defined meaning. I can define non-existing objects, such as "unicorn" or "hobgoblin" - that is to say, specify their connotation (pink horse with a horn on its forehead, for example) and observe that they denote a class that is empty (i.e. no unicorns exist outside the world of fiction, as far as we know).
But the main problem with the word "god" is not whether it denotes a class that is empty or not - but what is its connotation i.e. what it actually means. How do you define "god?" The definitions given by philosophers are already very vague and usually suffer from mixing epistemology with ontology i.e. whether the epistemological postulate of the "first mover" has any ontological equivalents, and the farther you move away from philosophy, the murkier it gets.
As an average intelligent person who professedly believes in god what exactly it is that he/she believes in. You will most likely get a blank stare, some may throw in a few meaningless synonyms like "creator" "supreme being" "great spirit" "force" etc. which convey certain emotional attitudes but say nothing what the object they supposedly denote is.
Stated differently, the word "god" is a reification of epistemological notion that our knowledge is necessarily limited, that there are things "out there" that may shape or lives (or may not) but exceed not just our current state of knowledge but human capacity to understand. If we keep it as an epistemological postulate - ignoramus et ignorabimus - which is the true meaning of agnosticism - we are fine. The problem starts when start leaving epistemology for ontology and start reifying, and then go back to epistemology seeking certitude.
Wojtek