Which C.S. Lewis book presents radio talks that became a major Christian apologetics work?

Mere Christianity presents C.S. Lewis’s radio talks as a major Christian apologetics work.

The book developed from talks Lewis delivered for the BBC during the Second World War. The broadcasts began in 1941 and were intended to explain basic Christian ideas to a broad audience, including listeners living through wartime uncertainty. Lewis revised and expanded the material for publication.

Mere Christianity is not a conventional autobiography or a novel. It argues from shared moral experience toward a general account of Christian belief, while avoiding detailed discussion of differences among Christian denominations. The title refers to what Lewis considered common or “mere” Christianity. The Problem of Pain is another apologetics book, but it addresses the problem of suffering rather than collecting the wartime broadcast talks.