Which C.S. Lewis book was based on his BBC talks about basic Christian belief?

Mere Christianity developed from C.S. Lewis’s BBC radio talks about basic Christian belief. The talks were broadcast during the Second World War, beginning in 1941.

Lewis was invited to address a broad British audience rather than a specialist theological readership. He discussed morality, belief, repentance, and the central claims shared by major Christian traditions. The broadcasts were later revised and collected in book form.

The title refers to what Lewis called “mere Christianity”: common ground among Christians before disagreements between denominations. The book therefore does not present itself as a complete account of one church’s doctrines.

Mere Christianity is often confused with The Problem of Pain, another Lewis work focused on Christian apologetics. The two books overlap in subject but originated differently: Mere Christianity grew out of radio addresses, while The Problem of Pain was written as a book.