Atheism worships a dead god.

[Entropist]

If, in thinking things through, we always have to account for even an infinitesimal possibility that some infinite god or other might pop into existence from behind a lamppost (please don't take me literally), then our thinking will be completely different from our thinking based on the certainty of gods' non-existence, which is why it is important to make the distinction.

I imagine that, like me, you would agree that reason is not the end-all and be-all of everything. Contra Aristotle, we are not rational animals. We are capable of using reason, as well as abusing it. Reason is only a tool-- a handy tool, which we are capable of putting to use. Reason is something we are capable of utilizing, but it is not inherent in any human nature.

That being said, one can never be absolutely certain of any god's existence or lack thereof (though the theist may claim he is absolutely certain, under the guise of "faith"). One can, however be reasonably certain, just as I am reasonably certain the sun will rise tomorrow-- but of course I am not absolutely certain.

We cannot be absolutely certain of anything-- only reasonably certain-- because reason is the only tool available to us that appears to get good results. Ancient books or political demagogues however are far less trustworthy.