There are two basic ways of asking this question. More common is "Pascal's Wager" and slightly less common is the notion that 'nobody can be 100% certain of anything'.
This boils down to saying something like, "If you're wrong, you'll go to Hell, so you'd better believe 'just in case'." It's admirably answered here, so I won't waste time covering that. (For grins, you might look at the Atheist's Wager, too.)
If you mean in terms of utter, complete certaintly... I'm not 100% sure that no 'god' of any type doesn't exist. Of course, in that sense, I'm not 100% sure that the sun will rise in the East tomorrow. Just because it's happened every day of my life that way, and (based on many lines of converging evidence from a wide variety of scientific fields) has happened that way for the last 4.5 billion years or so, who knows?
In the real world you don't get 100% certainty about anything. You have to use your best judgement and go with what you perceive the odds to be. Even in mathematics, things aren't really utterly certain. See, for examples, the recent proofs of the Poincare Conjecture or Fermat's Last Theorem. While the general consensus is that they have now been proven, the proofs are quite complex and there is still a chance a flaw may be found in one or both proofs.
(Now, some things are effectively certain. I mean, you can ask "What if we're wrong that 2+2=4?" but, aside from having fun in dorm-room bull sessions, there's no practical reason to consider that as any kind of realistic possibility.)
And note that in the case of, say, Fermat's Last Theorem, it would be easy to show that the proof is false. Just produce an example of x^n + y^n = z^n, where x, y, z, and n are integers and n>2. Of course, if the proof is correct, you won't be able to do so - and no one has ever been able to do so.
It would be easy to show that the 'argument from evil' is wrong somewhere, or at least substantially questionable - just give real, solid evidence that a God exists, like finding a counterexample for Fermat's Last Theorem. But, as has been mentioned before, that never really happens. Again I recommend reading Carl Sagan's essay on invisible dragons in garages, or ask why God never, ever, causes an amputee to spontaneously regrow a limb.
Based on the argument here, I'm about as sure as I can be that there's no 'God' anything like that of any religion I've heard of. Honest. Sure, I might be wrong, but this is really the way I figure it and nobody's pointed out any real flaws in the proof. I've been told that "maybe there is a God and It just has a really good reason that It can't prevent drunk drivers from accidentally killing babies, and It also has another really good reason for not telling us the first reason." (A paraphrase, yes, but I don't think it's materially different from what was actually said.) If that's the best anyone's got, well, I can't see why I should worry about going with my best judgement. If there's a God that would blame me for my honest appraisal of the situation, well, I can't see why I'd want to spend any time with It.
Just as a closing quote: "Question with boldness even the existence of a God; because, if there be one, he must more approve of the homage of reason, than that of blind-folded fear." -- Thomas Jefferson