Which C.S. Lewis book presents a fictional conversation about Christianity?

The C.S. Lewis book presenting a fictional conversation about Christianity is The Screwtape Letters.

Published in book form in 1942, The Screwtape Letters consists of letters from Screwtape, a senior demon, to his inexperienced nephew Wormwood. The letters discuss temptation, human weakness, prayer, pride, love, and spiritual distraction from a deliberately reversed point of view.

Lewis uses the demonic correspondence as a satirical device rather than as a straightforward theological dialogue. The imagined advice frequently exposes the flaws in the demons’ assumptions and draws attention to ordinary moral choices.

The work is sometimes grouped with Lewis’s direct Christian apologetics, but its format is distinctive. Mere Christianity presents arguments and explanations in a more conventional voice, whereas The Screwtape Letters uses fiction, irony, and an invented bureaucracy of demons.