Which C.S. Lewis work was adapted from radio broadcasts made during World War II?

Mere Christianity was adapted from radio broadcasts C.S. Lewis gave for the BBC during World War II. The talks began in 1941 and were later revised and collected into book form, first published in 1952.

Lewis designed the broadcasts to explain shared Christian beliefs to a broad audience rather than defend one denomination. He discussed morality, human nature, Christian doctrine and the meaning of faith in clear, conversational language. The title refers to the common core of historic Christianity.

The book is often mistaken for a direct transcript of the broadcasts. Lewis substantially edited and reorganized the talks, and the final volume combines material from several broadcast series. Its wartime origin helped make his voice familiar to millions of British listeners.