What is evil?

“He [Augustine] contends that good things can be corrupted, which leads him to this arresting conclusion: ‘I sought to know what wickedness was, and found it was no substance, but a perverse distortion of the will away from the highest substance and towards the lowest thing’ (Confessions 7.16.22, emphasis added). Which is to say that evil does not exist, but is only the diminution and distortion and perversion and corruption of the good. Or, again, as Plantinga puts it, ‘the person who curves in on himself. . . ends up sagging and contracting into a little wad.’ Do we say that sin ‘is’ or do we say that sin really is nothing more than diminution of God’s good things?”—A Community Called Atonement, p. 48 (emphasis his)