Which C.S. Lewis book was adapted from his 1941 BBC radio talks on basic Christian beliefs?

Mere Christianity was adapted from C.S. Lewis’s 1941 BBC radio talks on basic Christian beliefs. The broadcasts were delivered during the Second World War and were intended for a broad public audience rather than only for theologians.

Lewis organized the material around shared Christian ideas, moral reasoning, and the meaning of Christian belief. He later revised and expanded the broadcasts into a book, published in stages and then collected as Mere Christianity. The title refers to what Lewis regarded as the common core of Christianity, not to a separate denomination.

The work is often associated with the argument from morality and with Lewis’s famous illustration of a “Lord, liar, or lunatic” dilemma concerning Jesus’ identity. Readers sometimes assume the book began as a conventional manuscript, but its origin was radio. The talks helped make Lewis a widely recognized religious broadcaster in Britain.