What 1956 book by C.S. Lewis developed from his BBC radio talks about Christianity?

Mere Christianity is the 1956 C.S. Lewis book developed from radio talks he gave during the Second World War. The broadcasts were delivered for the BBC between 1941 and 1944 and addressed basic Christian beliefs.

Lewis aimed to present shared Christian ideas rather than defend one denomination’s distinctive doctrines. The resulting book is organized in four parts: “Right and Wrong as a Clue to the Meaning of the Universe,” “What Christians Believe,” “Christian Behaviour,” and “Beyond Personality.”

The title refers to what Lewis called “mere” Christianity: the common core of belief held across many Christian traditions. The book is sometimes mistaken for The Problem of Pain, another Lewis work about Christian belief, but that earlier book was published in 1940 and was not the direct compilation of the BBC talks.