What was the name of C.S. Lewis’s Christian apologetics book based on his wartime radio broadcasts?

C.S. Lewis’s Christian apologetics book based on his wartime radio broadcasts is Mere Christianity.

The book grew from a series of talks Lewis gave for the BBC during the Second World War. The broadcasts were later revised and collected in print, with the title Mere Christianity referring to the core beliefs shared across much of historic Christianity rather than to one denomination.

Lewis organized the material around moral law, Christian doctrine, and the implications of belief in Christ. His conversational style made difficult theological ideas accessible to a broad audience, helping the work become one of his most influential religious books.

Mere Christianity is sometimes confused with The Problem of Pain, another Lewis apologetics work, but the two began differently. The Problem of Pain was written as a book about suffering, while Mere Christianity developed directly from Lewis’s radio talks.