In his four-volume series of books "The Masks of God", Campbell tried to summarize the main spiritual threads of the world, in support of his ideas on the "unity of the race of man"; tied in with this was the idea that most of the belief systems of the world had a common geographic ancestry, starting off on the fertile grasslands of Europe in the Bronze Age and moving to the Levant and the "Fertile Crescent" of Mesopotamia and back to Europe (and the Far East), where it was mixed with the newly emerging Indo-European (Aryan) culture.

He believed all spirituality is searching for the same unknown force (which he spoke of as both an immanent and a transcendent force, or that which is both within and without, as opposed to only without) from which everything came, in which everything currently exists, and into which everything will return. He referred to this force as the connotation of what he called "metaphors", the metaphors being the various deities and objects of spirituality in the world. Any information to the contrary was regarded as "ridiculous" and ignored, so that what remains may be forcibly pushed into this worldview.