Which C.S. Lewis book developed from his wartime radio broadcasts about Christianity?

Mere Christianity developed from C.S. Lewis's wartime radio broadcasts about Christianity.

The original broadcasts were delivered for the BBC during the Second World War, beginning in 1941. Lewis spoke to a broad public audience rather than presenting a technical theological lecture, explaining common Christian beliefs in accessible language.

He revised and combined the broadcast material into books, which were later collected as Mere Christianity. The work is organized around themes such as morality, belief, Christian doctrine, and the meaning of conversion. Its title refers to the shared core of Christianity rather than to a particular denomination.

The book is sometimes confused with The Problem of Pain, another major Lewis work about Christianity, but that book was published in 1940 and was not formed from the famous BBC broadcast series. Mere Christianity became one of Lewis's most influential apologetic books.