C.S. Lewis first broadcast the talks later collected as Mere Christianity in 1941.
The talks were delivered for BBC radio during the Second World War. Lewis was invited to present short broadcasts explaining and defending central Christian ideas to a general audience. He continued the broadcast series in later years, and the material was revised and collected in book form.
Mere Christianity was not originally written as one continuous book. Its parts developed from broadcast talks given between 1941 and 1944, including material later associated with The Case for Christianity and Christian Behaviour. The final book appeared in 1952.
The title refers to what Lewis presented as shared or basic Christian beliefs rather than the distinctive doctrines of one denomination. Readers sometimes date the book itself to 1941, but that is the date of the first broadcasts; the assembled volume was published later.