Which C.S. Lewis book grew from his BBC radio talks on Christianity?

Mere Christianity grew from C.S. Lewis's BBC radio talks on Christianity. Lewis delivered the talks during the Second World War, beginning in 1941, and revised them into a book published in 1952.

The broadcasts were aimed at a broad British audience rather than an academic readership. Lewis presented what he regarded as shared or “mere” Christian beliefs before discussing specifically Christian claims. The book is divided into sections based on the talks, including “Right and Wrong as a Clue to the Meaning of the Universe” and “What Christians Believe.”

Mere Christianity is not a transcript of one uninterrupted broadcast series in its final form. It combines material from multiple wartime radio talks and later revisions. Lewis's The Problem of Pain, published in 1940, was a separate book and was not created from the BBC series.