God is light, and in him is no darkness at all. 1 John 1:5
Regarding evil and sin, Christian doctrine professes four essential truths. First, God is fully and continually all-powerful. Second, God is altogether good and there is no evil in him whatsoever.1Third, evil and sin really do exist. Fourth, sinners are fully responsible for their sin.
Various erroneous attempts to deal with evil do away with one of these truths and thus explain away evil or reduce the problem. Perhaps God is not all-powerful, or maybe God is not good, or maybe evil is an illusion. Maybe sin is not our fault but the fault of our parents’ failures or our circumstances. In opposition to these errors, the Bible—the most honest book ever written—faces evil in its thorough darkness without blushing or backpedalling and commands us to do the same. Rising up from the pages of Scripture is the greatest evil and sin of all, the murderous and bloody crucifixion of Jesus Christ.
We do need to make a distinction between moral evil and natural evil. Moral evil is the result of choices of a responsible agent, whether intentional or negligent. Natural evil is suffering that occurs without a moral agent involved (hurricanes, floods, earthquakes). Humans make no (or very few) actions causing natural evils.
Defining evil (the essence) and sin (the action) is very important. Among the most helpful thinkers in the history of Christian doctrine on this point is Augustine. Prior to his conversion to Christianity, he was part of a cult called Manichaeism. That cult—like many Eastern religions, pantheism, panentheism, and the New Spirituality (or New Age)—considered God to be both good and evil.
Augustine’s prayer in his book Confessions describes his own experience whereby God opened his eyes to his personal sin. Augustine prays: But You, Lord, while he was speaking, turned me back towards myself, taking me from behind my own back where I had put myself all the time that I preferred not to see myself. And You set me there before my own face that I might see how vile I was, how twisted and unclean and spotted and ulcerous. I saw myself and was horrified, but there was no way to flee from myself. . . . You were setting me face to face with myself, forcing me upon my own sight, that I might see my iniquity and loathe it. I had known it, but I had pretended not to see it, had deliberately looked the other way and let it go from my mind.2
Following his conversion, Augustine rightly said that evil was a flaw, a lack or deficiency in something inherently good. Evil is therefore a privation, or that which deprives a being of some good that is proper to that being. As a parasite, evil is all the more heinous because it destroys that which is beautiful and whole. Examples include blindness, which is not a thing in and of itself but rather a lack of sight, and rot, which is not a thing but rather the corruption of something like metal or wood. For this reason, Zechariah 10:2 uses the four words nonsense, lies, false, and empty to explain sin in terms of privation.
If someone asked you to define sin for them, how would you do so?