> I'd be interested in what kind of evidence Dennet would accept to show him that his atheism was wrong.

Not sure what you mean, any clear sign from a god would apply here. You seem to believe that such a thing isn't possible making their position irrational, but then I wonder how you can still say you believe in a god? Do you believe that God can't intervene in this world?

But for example, if God manifested giant talking heads all over the world I am pretty sure atheism would disappear very quickly.