Manichaeism & the Problem of Evil
The cause of sin or evil in the world has challenged the human mind through the centuries. The ancient Manichæns believed there were two powerful opposing deities. A powerful Principle caused all of the good in the universe while an opposing Principle caused all the evil. The battle of good and evil began when the Prince of Darkness decided to invade the Kingdom of light.
Before the existence of heaven and earth and all that is therein, there were two Principles, the one Good the other Bad. The Good Principle dwells in the realm of light and is called the Father of Majesty. .... Opposed to the Father of Grandeur is the King of Darkness. He is actually never called God, but otherwise, he and his kingdom down below are exactly parallel to the ruler and realm of the light above.1
Evil seems like a real thing in the universe, so it is natural to think there must be two opposing forces that cause the good and the evil. It does not make sense to think that good would cause evil, since good is contrary to evil. To think that good causes evil is similar to claiming that heat causes coldness or light causes darkness. It is a self-contradictory thought. Since we find abundant goodness in the universe, there must be a good cause for it. Evil is in the world too. So, it must have a cause that is distinct from the cause of the good. The Manichæns took the problem of evil seriously and adopted theistic dualism to resolve the the problem of good and evil by claiming there must be two opposing eternal Principles. They felt that a good deity could not provide an answer for the existence of evil.
The Manichæns believed that the King of Light was all-good (omnibenevolent). However, he was not all-powerful (omnipotent). This belief conflicts with the classic Christian view of Deity who is both all-good and all-powerful.
1 From Catholic Encyclopedia: Manichæism. For further information on the topic of Manicheaism, see the on-line article on Manichæism in the Catholic Encyclopedia.