• God is all-powerful
• God is all-good
• Terrible things happen

If you wish, you can replace that last one with "Evil things happen in the world".
Morrow proposed that one could accept any two of these as true, but not all three, citing several sources for that conclusion.  So if God is all powerful, and evil exists, than God can't be all-good.  Or God is all-good, but is not powerful enough to stop evil.  The third choice, that God is all-powerful and all-good but nothing terrible ever happens is obviously false, although there are people who believe that option saying that what we consider evil is just things we don't like, but they are really not evil or wrong because, if they were, God would put a stop to them immediately.  
    Gnostics have a different point of view.  We say all three are true.  Despite the existence of an all-powerful and all-good God, evil still exists and bad things do happen.  Why?  For several reasons.  The first of course is that we don't consider the material world to be the creation of God, but rather a creation of the fallen angels, Lucifer and his associates, so of course there is evil in it.  Second reason is our much valued freedom of choice.  When we make bad choices, we get bad results.  When we thing evil and violent thoughts, we generate evil and violence in our world.  God can correct this, and soon will one way or another, and the choice of how is essentially ours.  We can choose to start obeying the Will of God and help in His plan to turn the world back to it's original state of evil-free spirit, or we can refuse and God will finally abandon us for the fallen angels to control for good.  And if you think the world is evil know, it's mild compared to what will happen if God does abandon us completely.