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

Mere Christianity grew out of C.S. Lewis's wartime BBC radio talks.

Lewis delivered the talks in Britain between 1941 and 1944. Their purpose was to explain shared Christian beliefs in accessible language rather than defend the doctrines of one particular denomination.

The talks were later revised and published in book form. Mere Christianity is organized around moral law, Christian belief, and the virtues and doctrines that Lewis considered common to historic Christianity. Its title refers to the idea of a central or basic Christianity shared across denominational divisions.

The book is sometimes described as a transcript of the broadcasts, but Lewis edited and reshaped the material for print. The radio talks also helped make his voice familiar to a broad British audience during the Second World War.