Which C.S. Lewis book was based on radio broadcasts made during World War II?

Mere Christianity was based on radio broadcasts C.S. Lewis made during World War II. The talks were delivered for the BBC between 1941 and 1944 and later revised and published as a book.

Lewis designed the broadcasts to explain central Christian ideas in language accessible to a general audience. He discussed morality, belief, Christian doctrine, and the differences among Christian traditions, while deliberately focusing on what he regarded as shared Christianity rather than denominational disputes.

The book’s title came from the idea of a common or “mere” Christianity. It was not originally broadcast as one continuous book, and its sections grew from different series of talks. That publishing history distinguishes it from The Four Loves, another Lewis work connected with broadcasting but focused on forms of human love.