The book challenges the religious dualism of body and spirit, and Knowlton presents a psychological theory that's been described as "early behaviorism."

The religious dualism of Christianity is not a perfect dualism as God (good) will inevitably destroy Satan (evil).

Some scholars see contact with religious dualism in Babylon, and early Zoroastrianism in particular, as having influenced Second Temple period Judaism, and consequently early Christianity.

The prophet Mani, in the middle of the 3rd century in Persia, expanded this ethical interpretation of the continuing struggle between light and dark into a strong form of religious dualism; Manicheans envisaged the clash of forces as a personal, eternal struggle between divine powers of good and evil.