Which C.S. Lewis book developed his BBC radio talks into a defense of Christianity?

Mere Christianity developed C.S. Lewis’s BBC radio talks into a defense of Christianity.

Lewis delivered the original talks during the Second World War, beginning in 1941. The broadcasts were designed for a broad audience and explained basic Christian ideas in direct, conversational language rather than in the style of an academic theology lecture.

The talks were later revised and published in stages, eventually forming the book Mere Christianity. Its title refers to the common core of belief shared by major Christian traditions, rather than to a separate denomination.

The book is organized around themes including morality, Christian belief, and the doctrine of the Trinity. Lewis’s “trilemma” argument about Jesus—often summarized as “liar, lunatic, or Lord”—comes from this wider apologetic tradition, though the phrase is frequently quoted without its surrounding discussion.