A nut case is someone who believes in something you and the vast majority of others don't believe in. If he has lots of company in his mistaken beliefs, he's not a nut case, he's just wrong.

Nowadays, if you believe Zeus is the king of the gods, you're a nut case. 2500 years ago, even people who didn't believe in Zeus were willing to cut the followers of Zeus some slack, just because there were so many of them.

Of course, these definiitions will differ depending on which crowd you run around in. Unfortunately, even though there may be only one truth, human perception of truth is often decided by vote.