Which C.S. Lewis book originated as radio talks broadcast by the BBC?

Mere Christianity originated as radio talks broadcast by the BBC.

Lewis delivered a series of Christian broadcasts for the BBC during the Second World War, beginning in 1941. The talks were later revised and collected into the book Mere Christianity, first published in 1952.

The work presents what Lewis regarded as the basic beliefs shared by major Christian traditions. It is organized in three sections: “Right and Wrong as a Clue to the Meaning of the Universe,” “What Christians Believe,” and “Christian Behaviour.”

The title does not refer to a separate church or denomination. Lewis used “mere Christianity” for the common core of historic Christian belief. The broadcasts made his voice familiar to British listeners and helped establish him as a prominent Christian public intellectual.