Which C.S. Lewis book began as BBC wartime radio talks?

Mere Christianity began as BBC wartime radio talks by C.S. Lewis.

Lewis delivered the first series of talks in 1941, followed by additional broadcasts in 1942 and 1944. The talks were intended for a broad British audience during the Second World War and presented basic Christian ideas in accessible language.

After revision and expansion, the material appeared in book form as Mere Christianity. The title refers to the shared core of belief found across the major Christian traditions, rather than to one specific denomination.

The book is often confused with The Problem of Pain, another major work of Lewis’s Christian apologetics. The Problem of Pain was published in 1940 and was written as a book from the outset. Mere Christianity is the work directly developed from the BBC broadcasts.