What was C.S. Lewis’s wartime BBC broadcast series later published as Mere Christianity?

C.S. Lewis’s wartime BBC broadcast series later published as Mere Christianity was titled Broadcast Talks.

Lewis’s first BBC talks were broadcast in 1941 and 1942, when Britain was fighting the Second World War. The talks used direct, conversational language to explain Christian ideas to a broad radio audience. They were later revised and issued in book form, first as Broadcast Talks in 1942 and subsequently in expanded related volumes.

The material developed into the structure associated with Mere Christianity. The first section discusses morality and the idea of a moral law; later sections address Christian belief and Christian conduct. Lewis deliberately aimed at the shared core of Christian belief rather than the distinctive teachings of one church.

The title The Case for Christianity belongs to the second set of broadcast-derived material, published in 1943. Christian Behaviour was another related volume, published in 1943. These titles are therefore plausible but do not answer the question about the original first broadcast collection.