Dualism is a false theory which explains a given situation or domain in terms of two opposing factors or principles.
Metaphysical dualism asserts that the facts of the universe are best explained in terms of mutually irreducible elements. They are often considered to be mind and matter, or thought and extension (Descartes).
What interests us more is ethical dualism which asserts that there are two mutually hostile forces or beings in the world. One is the source of all good, while the other is the source of all evil. These two forces of good and evil are best developed in the Iranian religion of Zoroaster.
The universe becomes the battle-ground for these opposing beings, identified as light and darkness, or the sacred and profane. In Chinese thought the analysis of reality is defined in terms of yang and yin.
All this is radically opposed to the Christian faith, which recognises God as the only eternal and supremely good Being, from whom and through whom and unto whom are all things. It holds that Satan and his demons are fallen angels who were originally created good but rebelled against their Maker.
But Christian theology does not consider Satan to be ultimate and original; it also sees him ultimately excluded from the universe, which at the Consummation, will be made new, in which righteousness alone will dwell.