Which BBC radio-talk book made C.S. Lewis widely known as a Christian apologist?

Mere Christianity made C.S. Lewis widely known as a Christian apologist through BBC radio talks.

The book was assembled from broadcasts Lewis gave for the BBC during the Second World War. The talks were delivered in several series between 1941 and 1944, then revised and collected into book form. Lewis aimed to explain shared Christian beliefs rather than defend one particular denomination, which accounts for the title “mere Christianity.”

Its opening section develops an argument from moral awareness, while later sections discuss Christian belief, conduct, and the doctrine of the Trinity. The book is not a novel and is not the same project as The Problem of Pain, although both are apologetic works. Lewis’s calm radio style and accessible analogies helped make him a prominent religious broadcaster in wartime Britain.