C.S. Lewis called the original radio talks Broadcast Talks.
Lewis delivered the talks for the BBC during the Second World War. The first series was broadcast in 1941 under the title Broadcast Talks, and later series used the titles Christian Behaviour and Beyond Personality. The talks were eventually revised and collected as Mere Christianity.
Mere Christianity presents ideas that Lewis believed were shared across historic Christian traditions. It discusses morality, human behavior, Christian belief, and the meaning of conversion in a direct style intended for a broad radio audience.
The modern book title should not be projected backward onto every original broadcast. The first broadcasts had a different title, while Mere Christianity became the familiar name for the collected and revised work. This distinction helps separate the radio history from the later publication history.