Which C.S. Lewis book grew out of his wartime BBC broadcasts?

Mere Christianity grew out of C.S. Lewis’s wartime BBC broadcasts.

Lewis delivered the first series of talks in 1941, followed by additional broadcasts in 1942 and 1944. The talks were intended to explain basic Christian ideas to a broad radio audience during the Second World War.

He later revised and combined the material into book form. Mere Christianity was published in 1952 and became one of Lewis’s most widely read works of Christian apologetics. Its opening arguments discuss moral awareness and the idea of a common moral law before turning to Christian beliefs about God, Christ, and Christian conduct.

The title means the central beliefs shared across historic Christian traditions, rather than a separate denomination. Lewis was an Anglican, but the book is written as an attempt to present common ground rather than detailed denominational theology.