Which C.S. Lewis book began as radio broadcasts on Christianity?

C.S. Lewis's Mere Christianity began as radio broadcasts about Christianity. Lewis delivered the talks for the BBC during the Second World War, beginning in 1941.

The broadcasts were later revised and collected into book form. Mere Christianity was published in 1952 and presents a broad case for shared Christian beliefs rather than focusing on differences between denominations. Its sections developed from the broadcast series known as Broadcast Talks and related talks.

The book helped make Lewis a widely recognized public religious writer. Its opening discussion of morality and the “law of nature” became particularly well known. Although the book is often associated with the BBC talks, its final published text is an edited literary work rather than a verbatim transcript of every broadcast.