He is drunk.

He does not think that he is so drunk that he is not comprehending her words. So he believes that she has begun to speak of demons and resurrection and Athos can do nothing but gape. "I have had much to drink," he confesses, "but please forgive me if I do not believe you. I do not think I am hallucinating, but you are making a very strong case for my lack of sanity at the moment."

She cannot be truthful. Such things cannot exist. Can they?