Use Bertrand Russell's Celestial Teapot:
If I were to suggest that between the Earth and Mars there is a china teapot revolving about the sun in an elliptical orbit, nobody would be able to disprove my assertion provided I were careful to add that the teapot is too small to be revealed even by our most powerful telescopes. But if I were to go on to say that, since my assertion cannot be disproved, it is an intolerable presumption on the part of human reason to doubt it, I should rightly be thought to be talking nonsense. If, however, the existence of such a teapot were affirmed in ancient books, taught as the sacred truth every Sunday, and instilled into the minds of children at school, hesitation to believe in its existence would become a mark of eccentricity and entitle the doubter to the attentions of the psychiatrist in an enlightened age or of the Inquisitor in an earlier time.
Source:
http://www.cfpf.org.uk/articles/religion/br/br_god.htmlIf you ask him whether or not he believes there's a china teapot floating out around the orbit of Mars, it's a good bet that he'll say, "No."
Now ask him if he
knows, for sure, if there's a china teapot orbiting Mars. He'd be a liar if he said yes since (in so far as I know), there's no way for us to currently detect such a small object from Earth and the Mars orbiters don't cover the whole planet (nor am I sure if they could detect such a small object). So there's no way for him to know.
That's the difference between not believing and knowing that there's no god.
Of course, I'm sure once humans do reach Mars they'll find some asshole lab tech back on Earth, just for shits and giggles, has in fact launched a china teapot into orbit about the Red Planet.