The BBC radio series that introduced much of C.S. Lewis’s Mere Christianity was Broadcast Talks.
Lewis delivered the first series of radio talks during the Second World War, beginning in 1941. The broadcasts were intended for a broad British audience and presented basic Christian ideas in direct, accessible language rather than in the style of an academic lecture.
The talks were later revised and collected into books. Their successive published forms included The Case for Christianity, Christian Behaviour, and Beyond Personality. These parts were eventually brought together as Mere Christianity, the title by which the complete work became famous.
A common mix-up is to call the original broadcasts Mere Christianity. That name is associated with the later collected book, while Broadcast Talks is the title of the first published collection of the radio material.