I love this little piece of nonsense tossed around by the religious right (and left of course). The fact is, it's true. Yup, just because you have no evidence for something does not mean that you actually have proof that something does not really exist. I have no proof of god, but I can't say he doesn't exist. Or can I?
Let's use a simple little analogy. I have no proof that there isn't buried treasure in my backyard, but I can't say that means that there is no buried treasure in my backyard. Fair enough I believe, but what do we do about this little nugget of truth? Well, we do what any right-minded person would do; we dig for the treasure! We keep digging...and digging...and so on, until the backyard looks as if it were under attack by thousands of dogs trying to track down dinosaur bones (depending upon how deeply we dig of course).
Now, at this point I still have no evidence for treasure; I probably have evidence for one hell of a landscaping bill though. So what do I do? Well, absence of evidence is not evidence for absence! So let's keep digging!
How long do you think you would dig before you began to reach a conclusion that, based upon the assessment of my backyard, the probability of finding treasure was reaching zero. Let's also briefly touch on the Theory of Limits (calculus guys). The probability won't be zero, since you could always argue that the little patch in the corner could hide the treasure, but we're damn close to zero.
Christianity has had about 2,000 years to provide evidence of the existence of their god (well, one those gods anyway since they can't seem to agree on him/her). I'm not talking philosophy here, I'm talking about empirical evidence. I could wax philosophical all day about the logical presence of the treasure in my backyard, but what does that really amount to? Now if I found a doubloon in the flower bed....hmmm, evidence! I could formulate an hypothesis, test it, then reach a conclusion!
Religion has utterly failed to provide a single shred of substantiated evidence for the existence of a god. Does this disprove his existence? Not at all. But given all of that time, the Theory of Limits comes to mind; the probability of god is greatly reaching zero. How long would you dig in your backyard for treasure absent any evidence for it? My advice? If you found yourself in a hole, I would recommend you stop digging. The universe is a beautiful, amazing place. Don't fuck it up with superstition.