THIS WORLD OF OURS is a world of opposites. There is light and shade, there is heat and cold, there is good and evil, there is God and the Devil.
The dualistic conception of nature has been a necessary phase in the evolution of human thought. HOD, P1, Paul Carus
Dualism, in a nutshell, is the concept of a world of opposites, and usually implies or explicitly states that when you have one option, there will be opposite option. A Christian trite and weak explanation for the evil in this world is the dualist concept that freewill requires both the ability to do good and to do evil. Don't worry. Carus is no believer of dualism
Upon close inspection, it is clear that the concept of Dualism does not actually line up reality, at least not most of it. Physicists will tell you that there is not light and dark, but light and the absence of light, and there is not heat and cold, but heat and the absence of heat.
What is the opposite of green? You may think back to the color wheel for a conceptual understanding, but "green" is really nothing more than electromagnetic radiation with wavelengths of around 520–570 nanometers. So what is the opposite of 520–570 nanometers?
Dualism fails in our reality of continuous, and sometimes discontinuous (quantum), spectrums of manifested matter and energy. Not every relationship is polar. In fact, very few are. Dualism only works as a metaphor, an extremely simplified model. That said, even primitive models can be useful, as long as we never forget that they are just tools.
Unfortunately, when religions get to the stage of conceptualizing dualism, it is not considered a model, but rather an actuality. Carus goes on to explain the "necessary phase" in our development coming to be like so:
But the principle of unity dominates the development of thought. Man tries to unify his conceptions in a consistent and harmonious Monism. Accordingly, while the belief in good spirits tended towards the formation of the doctrine of Monotheism, the belief in evil spirits led naturally to the acceptance of a single supreme evil deity, conceived as embodying all that is bad, destructive, and immoral. HOD, P1, Paul Carus
Interestingly enough, the Judeo-Christian faith is an example where we can see this evolution to dualism in action. The God of the Old Testament originally wielded the forces of both good and evil, ultimately for His conception of the greater good, but the New Testament largely purges God of those evil roles, making Satan their supreme owner instead.
Christianity usually holds that the Serpent in the Garden of Eden was Satan, despite the obvious clues to the contrary. With that perspective, they miss the evolution completely, even though the Scripture is often explicitly contrary to a dualist interpretation.
There are several incidents which come to mind to support this evolution to Christian dualism, but for the sake of brevity, I'll leave you with one potent example. In Deuteronomy 13:1-3, we find:
If a prophet, or one who foretells by dreams, appears among you and announces to you a sign or wonder, and if the sign or wonder spoken of takes place, and the prophet says, “Let us follow other gods” (gods you have not known) “and let us worship them,” you must not listen to the words of that prophet or dreamer. The LORD your God is testing you to find out whether you love him with all your heart and with all your soul. NIV
Right there, in black and white (wink), in the Old Testament God tells us that He will send false prophets to lure the Israelites away from worshiping Him; tempting them to sin and testing their faithfulness to Him.
Now, in the New Testament, in James 1:13-14 we find:
When tempted, no one should say, "God is tempting me." For God cannot be tempted by evil, nor does He tempt anyone; but each one is tempted when, by his own evil desire, he is dragged away and enticed. NIV
So instead of God tempting people in the New Testament, Satan gets that job, as we see in 1 Corinthians 7:5:
Do not deprive each other except perhaps by mutual consent and for a time, so that you may devote yourselves to prayer. Then come together again so that Satan will not tempt you because of your lack of self-control. NIV