Which C.S. Lewis book presents Christian ideas through correspondence from a senior demon?

The C.S. Lewis book presenting Christian ideas through a senior demon’s correspondence is The Screwtape Letters.

Published in book form in 1942, The Screwtape Letters consists of letters from Screwtape, an experienced demon, to his inexperienced nephew Wormwood. Screwtape advises Wormwood on how to tempt a human patient and prevent that person from moving toward Christian faith.

Lewis uses the inverted viewpoint satirically: readers are expected to recognize the demons’ strategies as distortions and failures. The book discusses distraction, pride, social pressure, fear, and spiritual complacency without presenting the advice as a literal handbook for wrongdoing. Its epistolary structure gives the theological argument a distinctive fictional form.

The work is sometimes confused with Lewis’s 1941 radio broadcasts later collected as Mere Christianity. Both address Christian belief, but only The Screwtape Letters uses a series of fictional demonic letters.