As scripture has it, God did a lot of creating before he brought humankind into being. Long before, he created heaven, hell, angels, and of course, the devil.
Ever wonder why would God do something like creating Satan? If God is all-powerful, then Lucifer's fall from grace could have easily been averted. So God must have wanted this to happen. Therefore, God is obviously responsible for all evil in the world.
God causes everything to happen, every atrocity, as well as every nicety. Everything is supposedly according to God's plan, whatever that may be. Seems to me that his plan is to play with us as a child plays with dolls. He moves us around, puts words in our mouths, and makes us live the scenarios in his mind.
Is this what people think of when they say "God?" Most likely, no. But that's what an omnipotent, omniscient being would be. And that's why the concept is so preposterous. There are lots of other problems with the biblical God that add to the lunacy of the concept, but even without those, it's enough for me to realize what a ridiculous notion an omnipotent Creator is. What truly boggles my mind is why the majority of people in the world don't see this obvious fact.
More's the pity.
