Which C.S. Lewis book began as BBC wartime broadcasts in 1941?

Mere Christianity began as C.S. Lewis’s BBC wartime broadcasts in 1941.

The BBC invited Lewis to give radio talks during the Second World War, and the first series was broadcast in 1941. Further talks followed in 1942 and 1944. Lewis addressed ordinary listeners rather than presenting a technical academic theology, discussing morality, Christian belief, and the shared core of Christianity.

He later revised and combined the broadcasts into the book Mere Christianity, published in 1952. The title refers to the common beliefs shared by the major Christian traditions, not to a separate denomination. Because the final book was published more than a decade after the first broadcasts, readers sometimes mistake 1952 for the date when the project began.