Secure in their ignorance, these simple folks really believe this is a killer argument.
I will set aside Christopher Hitchen's quip, "That which can be asserted with no evidence may be rejected with no evidence", even though it does cut straight to the heart of the matter.
Rather today I want to examine their claim in a somewhat differing light, offering an example which I hope will assist even the truest of believers in recognizing the thinness of the ice on which they have chosen to skate.
First a lesson in basic logic. Existential statements, those asserting that there is a (non-mathematical) entity possessing a specific property (or set of non-contradictory properties) can never be conclusively disproved...that is just the nature of any such proposition. Should theists therefore take comfort in this fact? Can they?
The answer is a resounding no, and where my example comes into play, because no matter how absurd a claim for existence is it can never be completely disproved.
I can claim, and I hereby do, that somewhere on our planet there exists a dog which can and does speak perfect English out of its butt, and I challenge theists (or anyone else for that matter) to prove me wrong. Before you injure yourself attempting to jump through logical hoops however, permit me to assist you. It cannot be done.
No one can look everywhere, check every dog, and say with total authority that I am wrong because there is always the probability that they have missed something somewhere or even, in this case at least, come across a butt-speaking dog which was not in the mood to cooperate.
Although they cannot be disproved, existential claims can on the other hand be proved quite simply. In the case above, all I need to do to prove my claim is to present the dog in question and the argument is over.
And so it is, my theist friends, that I need to ask: Where, precisely, is YOUR butt-speaking dog?