proximoception

It's not a question of likelihood. The existence of God is not a probability, and we don't really communicate anything when we tell people that his existence is extremely unlikely. The concept of a God, and of purely metaphysical entities in general, establishes an order of logically possible beings that are by definition epistemologically inaccessible. To that extent it would not only be superfluous to say anything additional about them or their properties (existence included); to do otherwise would be to commit an error which is effectively similar to those made by theists, that is, to claim knowledge of that which by definition can never be known.

And of course, it's perfectly rational not to believe in what is epistemologically inaccessible. And again, the problem here is not one of probability. We need not say that this disbelief be absolute (as if there were a chance that we could know whether or not God existed -- there isn't) in order for disbelief to be valid. But you get into trouble when you go from claiming "I do not believe in God" to "God does not exist". If you accept using convention as a standard -- and if I interpret the end of your last message correctly, you do -- then I think we can agree that most atheists would not limit themselves to the former claim, no?