What was the title of C.S. Lewis's 1950 book based on wartime radio broadcasts?

C.S. Lewis’s 1950 book based on wartime radio broadcasts was Mere Christianity.

The book developed from a series of BBC radio talks Lewis delivered during the Second World War. The talks were broadcast in three series between 1941 and 1944, and their material was later revised and published in book form. Lewis presented arguments about morality, belief, and shared Christian ideas rather than focusing on one denomination.

Mere Christianity is divided into four parts: “Right and Wrong as a Clue to the Meaning of the Universe,” “What Christians Believe,” “Christian Behaviour,” and “Beyond Personality.” Its title refers to the common core of Christian belief that Lewis wanted to discuss.

The book is distinct from The Problem of Pain, an earlier 1940 work about suffering, and A Grief Observed, Lewis’s later reflection on bereavement. Those books are often grouped with it but had different origins and purposes.