How do you make a short and simple explanation of the existence of evil? Here are some suggestions:
First, you note that the problem of evil is a problem for all mankind. It is not a uniquely Christian problem. Reasonable persons of every worldview must explain why evil exists in the world. Eastern religions only escape it and ignore it and pretend that there is no enemy anywhere. That works well until an assailant wants to rape your daughter, and does so right in front of you, or steals your car. No enemy?
A second point you can make is that even an unbeliever must explain the existence of exceptional good, too. Why do humans make unbelievably harrowing sacrifices to save lives? Why are there gorgeous sunsets? Why does the adorable chortle of a newborn baby sweep over us with gladness? G. K. Chesterton, the venerable British journalist (from the same milieu as C.S. Lewis) observed that these realities are evidence of a shipwreck. These beautiful serendipities are like pieces of expensive jewelry that fell out of a broken treasure chest on the beach. The jewels strongly suggest that there is a story somewhere bigger than our current existence…somewhere else that I came from, and somewhere else where I’m heading eventually.
Third is the observation that God has told us true things in the Bible, but He has not told us everything. Biblical revelation is true knowledge but it is not exhaustive knowledge about our existence. Now we only know in part. We all must make our peace with that—that we are not God, and that His mind and purposes are bigger than ours. For the most part He has been silent on the big-picture issue of evil’s existence. Nonetheless, we can note that He is not altogether silent on the topic. He has declared two things about it: one with His words and the other with His body. His words warn us to not participate in the works of evil, and the Bible commands us to not devise evil.
In addition, He came down and personally endured the reality of evil in this world. By laying down His body on the cross of crucifixion He showed that He is not distant on the issue. He endured in His own body punishment for—and by—the works of evil men, including the men who crucified Him without a just cause. Christ came down and willingly submitted to and participated in our suffering, Himself, to show us that He cares, that He knows our hearts will break, and that our lives will be afflicted.
Through His willing sacrifice of His own body, the Savior declared that He is our companion in our sorrows and griefs. He is not a far-removed God. He took outrageous slings upon Himself and underwent its horrors, to declare that “I am with you in whatever befalls you. I cannot tell you now why evil exists, but I can walk through it with you.” No other (man-made) god or human potentate has ever been willing to do such a thing, staying so near, so lovingly involved with us.
A wise theologian predicted that in the end, when we finally have seen the reasons for allowing a season of evil (in addition to all of the other perplexities we undergo in this small chapter of earth-life), we will agree with God. There is clearly and surely a lot that we don’t know and cannot know while in this state. We must trust God in the dark, based on what we have learned about Him in the light. The only alternative is to trust our limited finite selves (we, who cannot even hold a whirling electron to a neutron in an atom). In doing so, we lean upon a broken reed.