Which C.S. Lewis work adapts his BBC wartime broadcasts into a book of Christian apologetics?

Mere Christianity adapts C.S. Lewis's BBC wartime broadcasts into a book of Christian apologetics. Lewis delivered the talks between 1941 and 1944, and they were later revised and collected in book form.

The broadcasts were designed for a broad radio audience during World War II. Lewis discussed morality, Christian belief, and the central ideas shared by major Christian traditions. He did not write the talks as a denominational manual, and the title refers to what he presented as common Christianity.

The book was assembled from earlier works, including Broadcast Talks and Christian Behaviour, and was published in its familiar combined form in 1952. It is distinct from The Problem of Pain, which Lewis published in 1940 as a separate discussion of suffering and belief.