Which radio broadcast series by C.S. Lewis was later revised into Mere Christianity?

Broadcast Talks was the radio series by C.S. Lewis that was later revised into Mere Christianity. The BBC broadcast the talks during the Second World War, beginning in 1941.

Lewis was invited to explain basic Christian beliefs to a broad radio audience. The talks were designed for listeners who might not belong to one particular Christian denomination. After the broadcasts, Lewis revised and expanded the material for book publication, combining the talks into a single work on shared Christian belief.

The later sections originally appeared under related titles, including Christian Behaviour and Beyond Personality. The final book, Mere Christianity, was published in 1952. The title does not mean a minimal or diluted religion; Lewis used “mere” in the older sense of basic or essential Christianity shared across traditions.