Which C.S. Lewis book presents Christian ideas through fictional conversations between a senior and junior demon?

The C.S. Lewis book presenting Christian ideas through conversations between demons is The Screwtape Letters.

The book is structured as a series of letters from Screwtape, an experienced demon, to his inexperienced nephew Wormwood. Screwtape offers advice on tempting a human subject, but his instructions reveal the moral and spiritual errors Lewis wanted readers to recognize. The unusual viewpoint creates satire by describing virtue, prayer, humility, and faith from an enemy’s perspective.

The letters were first published in book form in 1942 after appearing in the Anglican newspaper The Guardian. Lewis wrote them during the Second World War, and the work quickly became one of his most widely read books for adults.

The Screwtape Letters is not a conventional novel with a continuous adventure plot. It is also distinct from Mere Christianity, which develops Christian arguments more directly, and The Great Divorce, which uses an imagined journey to explore heaven, hell, and choice.