See, Lewis' satire is a reminder of something God iterated over and over in the Bible: the devil is real.
There are some things that we as Christians like to gloss over. This is one of them. I mean, who wants to talk about someone whose entire existence revolves around ruining - and taking - our lives? Some would consider that inappropriate discussion for the dinner table.
The reality though, is that we need to discuss the devil and his minions. As Lewis said in the preface to The Screwtape Letters, "There are two equal and opposite errors into which our race can fall about the devils. One is to disbelieve in their existence. The other is to believe, and to feel an excessive and unhealthy interest in them. They themselves are equally pleased by both errors."
Satan was kicked out of Heaven because he imagine himself higher and better than God, and he is still operating under this ridiculous pretense. If you read the list of goals taped to his mirror you'll see the opposites of everything God plans to do. God has plans to give us "a hope and a future." The devil wants to steal everything that might give us joy. Jesus came to give "life... and more abundantly." The devil comes "to steal, and to kill, and to destroy." God loves us unconditionally. Satan hates our guts.
The devil's main weapon is lies. John 8:44 says, "He was a murderer from the beginning, not holding to the truth, for there is no truth in him. When he lies, he speaks his native language, for he is a liar and the father of lies." He takes the Bible's truth and twists it around. He deceives us by influencing those around us. He toys with our emotions. Anything to steal us away from Father God.
But here's the thing: he can't. Jesus himself said in John 10:29, "My Father, who has given them to me, is greater than all; no one can snatch them out of my Father’s hand." For any temptation offered us, Jesus promises a way out. He cast out demons, and he gave us authority to do the same. All authority in Heaven and on Earth! He has given everything we need to kick Satan's butt. Just check out Ephesians 6:10-18. The Bible says, "Resist the devil, and he will flee from you." And always remember that there is power in the name of Jesus. Satan has no chance against the power of God that is in you!
So yes, the devil is real. Yes, he is alive. Yes, he "prowls around like a roaring lion looking for someone to devour."
But. He also sucks. And he's a loser. And a coward. So bam.
more scriptures about the devil's poopy-ness:
1 John 3:8
1 Peter 5:8