Which Christian apologetics book by C.S. Lewis grew from his wartime BBC radio talks?

Mere Christianity grew from C.S. Lewis’s wartime BBC radio talks. The talks were broadcast between 1941 and 1944 and later revised into the book published in 1952.

Lewis designed the broadcasts to explain and defend basic Christian beliefs in language accessible to a broad British audience during the Second World War. The book is organized around themes such as morality, Christian belief, and the doctrine of the Trinity.

The title refers to what Lewis considered common or central Christianity shared across many denominations, rather than to one particular church tradition. That approach helped the work reach readers beyond the audience of his academic publications.

Mere Christianity is sometimes confused with The Problem of Pain, another Lewis apologetics book. The earlier work focuses especially on suffering, while Mere Christianity developed from the radio talks and presents a broader case for Christian belief.