I think you make a slight and subtle error in your interpretation of Campbell's critique of Christianity. Campbell is, of course, pointing out the misinterpretation of symbols by the church and his criteria for a proper interpretation is based on the syncrity of human thought and jungian notions of the archetypes. His argument is simply that the efficacy of the symbol as a device of reconcilliation to our existence is diminished in any attempt to make that symbol an historical fact. It is a rather simple and yet profound idea that asks the reader to "believe" nothing. Campbell would argue that the genius of Christianity, that is turning reconcilliating archetypes into historical realities, is ultimately self defeating.