This sort of thing annoys me; why are so many Christians bound and determined to pretend that everybody they admire must be Christian too? Abraham Lincoln, Albert Einstein, Stephen Hawking, so many men have been classified as Christians against their will and against the truth, and now we can add Joe Campbell to the list.
Here's a quote from Joe Campbell which should help put that to rest:
From "The Hero's Journey".Joe Campbell wrote:God is a metaphor for a mystery ... half the people in the world are religious people who think that their metaphors are facts. Those are what we call theists. The other half are people who know that the metaphors are not facts and so they're lies. Those are the atheists.