Lewis talks about free will. He says that there is evil in the world because people have the freedom and the capacity to choose to do bad things over good things. But free will is the only thing that makes genuine love, peace, and joy possible. God deemed it necessary to give us free will.
Another reason why there’s evil: we have a tendency to put ourselves first rather than other people or God, and that has disastrous consequences, because true goodness, happiness and joy can only be found in God (As St. Augustine said, “You made us for Yourself, O God; our hearts are restless until it rests in Thee.”), so the moment we go against His will and follow ours, we set ourselves for misery and ultimately death, whether physical or spiritual.
God gave us free will, but we messed up. Still, He pursued us. He gave us conscience, good dreams, prophets and miracles, and His only Son, Jesus Christ.
The “shocking” thing that Lewis is referring to is the fact that Jesus is this very God who is the creator of the universe and the source of the Moral Law. He spoke in God’s place, claimed to forgive sins, and performed miracles and exorcisms. He was either a liar, a lunatic, or He was the Lord.
Christians believe that Jesus Christ really is the Lord. Aside from performing miracles and exorcisms, and forgiving people’s sins, He ultimately demonstrated His deity by rising from the dead following His execution.