Which C.S. Lewis book presents Christianity through conversations first broadcast on BBC radio?

The C.S. Lewis book based on BBC radio conversations is Mere Christianity.

Mere Christianity developed from a series of BBC radio talks Lewis gave during the Second World War. The broadcasts were intended to explain basic Christian beliefs to a broad audience, including listeners who did not belong to a particular denomination. Lewis later revised and combined the talks into a book, first published in 1952.

The title refers to the shared core of belief found across much of historic Christianity, rather than to a separate church or denomination. Lewis discusses morality, belief in God, Christian doctrine, and the meaning of Christian life. The radio format influenced the book's accessible, conversational style.

A common mix-up is to identify The Problem of Pain as the BBC work. That 1940 book addresses the intellectual and emotional problem of suffering, whereas Mere Christianity grew specifically from wartime broadcasts. The talks helped make Lewis a prominent public Christian writer in Britain.