Given that dualism is represented by the relationship between good and evil, as long as there has been something labeled as "good," something else had to conflict against it and was, therefore, called evil.
Arguments which exist in the supporting of dualism are grounded in the fact that there are two "sides to every coin." For example, both light and dark, good and evil (as mentioned before), and mind and body all exist as accounts of dualism.
As for in religion, dualism illustrates the differences between Western religions and Eastern religions. In regards to dualism within religion, the main areas where dualism is prominent are those which detail order and disorder, constructive and destructive, and objects/ideologies of opposition.
Basically, dualism exists within the confines of religion in order to define the idea that God/god balances the world given the existence of opposites.