C.S. Lewis' second obstacle to faith was: The Problem of Evil: How can I believe in God when there is so much evil, pain and suffering in the world? Isn't that inconsistent with an all-good, all-powerful God?
The problem of evil is perhaps the greatest of all obstacles for people considering faith in Christ. It was for Lewis. For Lewis, evil was both an intellectual and an emotional problem. He dealt with the intellectual problem in The Problem of Pain and with his own emotional struggle in A Grief Observed.
Of course, the problem of evil is not unique to Christianity. Every worldview or philosophy has to deal with why suffering exists. Lewis did not dodge the issue, nor can we.
Lewis saw us living in a good world gone wrong. If evil was real (which it is), then there must be an absolute standard by which it is known as evil. There must be an absolute good by which evil can be distinguished from good. Doesn't this demand a God as an adequate basis for absolute good? We feel that there are many things in this world that ought not to be the way they are. Our experience tells us that this is a good world gone wrong.
Eventually, Lewis saw the existence of evil as an argument for God's existence. An all-powerful, all good God created the universe. God has permitted evil and has a good reason for doing so. Therefore, there is no contradiction in theism.
God did not create evil, but he did create within humans beings the capacity to choose evil. While the capacity to choose evil is not evil itself, it provides the possibility for evil to be chosen.
As a young boy, Lewis lost his mother to cancer and later in life he lost his wife, Joy, to the same disease. Yet Lewis once said, "God whispers to us in our pleasures, speaks in our conscience, but shouts in our pains: it is His megaphone to rouse a deaf world." Pain topples the house of cards of our own sufficiency, forcing us to stop and examine ourselves.
Intellectual answers can never give specific reasons for why God permits the particular evils we encounter. Job never received an explanation for his sufferings. Instead, after a long silence, God asked a series of questions to show Job the limitations of his own understanding. Lewis came to the place where he not only believed in God, but chose to trust him even when he didn't understand God completely and despite his anger at times of suffering and pain. We can too.
[Summarised from chapter 4 of C.S. Lewis' Case for Christ]
Next: Objection #3 - Myth.