Which BBC radio talks by C.S. Lewis were later revised into a book?

C.S. Lewis’s BBC radio talks were revised into the book Mere Christianity.

Lewis delivered the broadcasts during the Second World War, beginning in 1941. They were intended for a broad audience rather than an academic one, and they presented what Lewis called “mere Christianity”: beliefs shared across many historic Christian traditions.

The talks were later adapted into three sections: “Right and Wrong as a Clue to the Meaning of the Universe,” “What Christians Believe,” and “Christian Behaviour.” The material was edited and published in book form in 1952, after earlier collections of the broadcasts had appeared.

Mere Christianity is not a denominational handbook and does not attempt to settle every disagreement among Christian churches. Its enduring appeal comes from Lewis’s conversational style, analogies, and emphasis on moral reasoning. The title is therefore sometimes misunderstood as meaning a reduced or less serious form of Christianity; Lewis intended it to mean the shared central tradition.