AGAIN, how you react to this hinges on whether or not you think what he's stating is actually TRUE.
Try it on the other way around.
Person X is simply saying that God does exist. If you believe that God does not, then you are wrong, that is, suffering from a delusion. Therefore all atheists are delusional.
Is this a statement which which you merely disagree, or do you find it offensive?
The reverse statement is not offensive to me. But I'm only speaking for myself. I'm no more offended by it than I would be offended if someone told me the mountains to the west of me didn't exist. That is, I would consider it symptomatic of something wrong with them, but I wouldn't take offense.