In his book A World of Difference, Kenneth Samples offers this anecdote. After attending a debate on the question "Does God Exist?", Ken had the opportunity to meet and talk with the atheist supporting the negative position. Here's his recount of the exchange:
I approached the atheist and shook his hand, thanking him for his efforts. Then I asked if it were correct to define atheism as the claim that "no god or gods are real" or that "no god or gods actually exist."
After some quibbling about the exact meaning of certain terms, the atheist essentially agreed that these two statements accurately reflected his position. I then asked, "If atheism asserts that 'no god is real' or that 'no god actually exists,' then isn't it making a universal claim about 'all reality' and 'all existence'?"
"In other words, as a point of logic,' doesn't the atheist, for his claim to be real, have to know all about reality and existence to rightly exclude any and every god. For example, to claim with any validity that there are no entities of a particular type (gods) in a given circle or set (reality), doesn't a person need a complete, comprehensive knowledge of that circle or set (reality)?’
I concluded my remarks by asserting that the atheist position could be valid only if atheists could justify their implicit claim to have a comprehensive knowledge of all reality and/or all existence. This position of seeming omniscience is, of course, beyond the capacity of any human being.
The atheist responded by saying that an incoherent god could not exist regardless of humanity's limited knowledge.
"That may well be true," I replied, "but then in order to maintain one's atheism a person must bear the burden of showing that every conceivable concept of God, is actually incoherent. This feat seems beyond the atheist's capacity."