In the historical-religious meaning of the term, dualism may be defined as that doctrine, formulated systematically or expressed in mythical form, which holds to the existence of two principles that, in different ways, have given rise to everything that exists and manifests itself in this world. This historical-religious notion of dualism thus concerns the ontological level of the creation and differs from the philosophical meaning of the term dualism as opposed to monism and equivalent to the affirmation of transcendence and from its use in an ethical sense the opposition between good and evil. A distinction is made between an absolute dualism, when the two principles are coeternal and mutually independent, and a mitigated or monarchian dualism, when the second principle derives from the first, but constitutes a principle by giving rise in turn to a creation of its own. An example of absolute dualism would be Iranian Zoroastrianism, esp. in the later formulations of the Middle Persian treatises, which oppose Ohrmazd old Avestic Ahura Mazda, the good creator god, to Ahriman = Angra Manyu, Destroying Spirit, the antigod, associated with lying and corruption and who, with his demons, penetrates the world to bring it death and destruction. Moreover, he operates a malevolent counter-creation of his own noxious animals, etc.. Iranian dualism is thus procosmic creation is a divine work and eschatological, affirming the final destruction of the evil principle and its deathdealing activity.
Examples of mitigated dualistic doctrines could be some systems of gnosticism of the first centuries which, starting from a supreme deity, show the origin, via a process of successive emanations and degeneration, of one or more persons the Demiurge, Archons of malevolent nature, who become creators of the world and of the human body, molding a matter held to be ontologically negative. The gnostic systems were happy to identify the inferior Demiurge with the God of the OT, opposing him to Jesus’ Father, who is totally unrelated to matter and the highest expression of that spiritual, divine world from which the human spiritual component, which the Savior came to redeem, is derived. The Fathers argued at great length against gnostic dualism, affirming the unicity of God, the good and just creator and savior of humanity, and the unity of revelation of the OT and NT.