C.S. Lewis began broadcasting the BBC talks later adapted into Mere Christianity in 1941.
The first series of talks was broadcast during the Second World War, when Britain was facing sustained military and social pressure. Lewis spoke about basic Christian beliefs for a general radio audience rather than presenting a technical theological course. The talks were followed by additional broadcast series in 1942 and 1944.
Lewis later revised and combined the material into the book Mere Christianity. The title refers to the common core of belief shared across historic Christian traditions, rather than to a separate church or denomination. The book became one of Lewis’s best-known works of Christian apologetics.
The broadcasts are sometimes dated only to the 1940s, but 1941 is the key starting year. The book itself was assembled and published later, so the date of the first radio talks should not be confused with the publication date of Mere Christianity.