Just like you 'know' him based on alleged conversations with this big guy in the sky? You're opinion about this god is based on a leather bound book, parchament, whatever while those who are areligious, can look objectively at the 'nature' of various gods, because they're not so enamored with this higher power. They can see the acts that his 'dictated' religious text has inspired, what it stands for, without trying to mush it up so that it fits with the rest of everything in that book. They can point out the errors and contradictions.
It's best when they remain open to all possibilities, however. Do we KNOW there is no god? No, but if there is, I believe the Christians, Islam, et al. have it all wrong - religion as we know it now is man-made for man's comfort. I can't say there is no god, just like I can't say we're not some space creature's experimental universe -- I leave all possibilities open for legitimate and hopefully eventually verifiable information.