I'll take a shot at defending the (heretical) proposition I mentioned below: that the existence of perfect good requires at least the potentiality of perfect evil. This proof occured to me around last Christmas vacation.
The argument below suggests to me, in the forward direction, that there will never be a time when an evil power does not exist. The evil power must be co-eternal with God in that sense. In the backward direction, too, it seems that the power must always have existed; otherwise there would be no being that could have always had the potentiality to be evil. Whether or not the evil power was always evil, is a different question.
1) Let's start by defining God as a being more perfect than whom nothing can be conceived, as Anselm did.
2) Now, perfection in all senses implies perfection in the moral sense as well.
3) To be morally perfect means giving all things their ordinate affections: loving each thing worthy of love, and hating each thing worthy of hatred, in their own measure.
4) Assume there was a time when the evil power did not exist. Then there would have been a time, before the universe began, that no evil thing existed.
5) At this time, there would have been nothing deserving of hatred, and hatred of evil would have been missing from the attributes of God, since one cannot hate something that does not exist.
6) God would then have the attribute of perfect love for that which is deserving of perfect love (in this case, the love would be the love of the persons of the Trinity for each other.)
7) But it is possible to imagine another, hypothetical, God who not only loved the perfect good with perfect love, but also hated and opposed the perfect evil with perfect hatred.
8) This God would have at least one more positive attribute (ordinate love + ordinate hatred, in comparison with simply ordinate love) than the God who had no perfectly evil being to hate.
9) This God would be more perfect than the real God. But this is a contradiction, since the real God is by definition the most perfect being we can imagine.
10) Therefore, assumption 4) was false, and there was never a time when the evil power did not exist, and there shall never be a time when the evil power does not exist. QED.
Put more simply, we consider it a good attribute to hate that which is worthy of hatred, and to oppose that which is worthy of being opposed. My Oxford Edition says this (that hatred of evil is the counterpart to love of the good) in the gloss on Revelation 2:6; But this thou hast, that thou hatest the deeds of the Nicolaitanes, which I also hate. God is a being who not only possesses all good attributes, but must have possessed them in all possible worlds. God cannot do that which diminishes His nature. Since God must always possess all good attributed, he must always possess the attribute of hating and opposing evil, which means evil must always exist.
Anyone want to take a shot at refuting this?