Which C.S. Lewis work adapts his 1941 radio talks on Christianity?

Mere Christianity adapts C.S. Lewis’s 1941–1944 radio talks on Christianity.

The broadcasts were delivered for the BBC during the Second World War and were later revised and collected in book form. Lewis addressed broad Christian ideas such as morality, belief, sin, and the nature of Christian faith, aiming at a general audience rather than a specialist theological readership.

The title refers to what Lewis presented as shared or “mere” Christianity across major Christian traditions. It is not a denominational handbook and does not focus on one church’s distinct doctrines. The book is sometimes confused with The Problem of Pain, another Lewis work of apologetics, but the two began as different projects.