Which C.S. Lewis book was based on his BBC radio broadcasts from 1941 to 1944?

Mere Christianity was based on C.S. Lewis’s BBC radio broadcasts from 1941 to 1944. The broadcasts were made during the Second World War and aimed to explain shared Christian ideas in language accessible to a broad British audience.

Lewis first published the material in separate books, including Broadcast Talks and Christian Behaviour. Those sections were later revised and combined into Mere Christianity, which became one of his most widely read works of Christian apologetics.

The book is not a denominational catechism. Lewis presents what he calls “mere Christianity,” meaning beliefs held in common across major Christian traditions. That purpose distinguishes it from The Four Loves, The Problem of Pain, and his fictional works.