Although he was well known as an authority on spiritual traditions, Paul Carus was unusual in that he refused to align himself with any particular form of organized religion. In any case, his universalist view of God would probably have put him outside the allowable boundaries of Christian belief. Carus was particularly active in raising awareness of Eastern spiritual traditions in the West.

His monumental study of the history of the Devil and other personifications of evil was published in 1900. Carus’ primary thesis is that the origin of religion is fear of the unknown and that the focus of primitive worship was on placating a hostile and vengeful deity rather than praising a beneficent form of divinity. As civilizations became more sophisticated, it was accepted that the forces of good would inevitably triumph, and the power and status of the evil forces gradually diminished. However, the devil, who has become in more recent times a demoted deity, still remains a source of fascination for religious and non-religious people alike.