Mere Christianity began as a series of radio broadcasts that C. S. Lewis ("Jack") gave on BBC Radio in London, England. Here is how A. N. Wilson describes the genesis of the work in C. S. Lewis: A Biography (New York: W. W. Norton & Company, 1990):
Feeling sheepish about his ineligibility for active service [in the early 1940s], Lewis had accepted an invitation from the RAF chaplains to tramp around the country and give talks to the men in various RAF stations. . . . The talks which Lewis gave to the RAF were on such basic issues as ‘Why we think there is a Right and Wrong’, and from such simple beginnings he framed, in language which was meant to be arresting to ordinary men in the ranks, an exposition first of the theist position, then of the Christian religion. In February 1941, he was approached by the Director of Religious Broadcasting at the BBC and asked if he would be prepared to give a series of broadcast talks on ‘The Christian Faith as I see it—by a Layman’; and although there was first a certain amount of debate about what the nature and title of the broadcasts should be, Lewis began to do this in the late summer of 1941, taking the train from Oxford to London every Wednesday evening, and broadcasting from 7.45 to 8.00 p.m.
The BBC had such a good response to Lewis’s [initial] four talks that they invited him back for a second series in which he moved to the next logical step. He discussed “What Christians Believe.” He did not expound Protestant or Catholic doctrine. He did not discuss mode of baptism or church government. He did not discuss how to calculate the return of Christ through the prophecies of Ezekiel and Daniel cross-referenced with Revelation. He spoke about what all Christians believe. He focused on the common faith that unites Christians. He gave reasons for why Christians believe what they believe about God entering into the world in physical form as Jesus of Nazareth. He explained why it makes good sense. Again, the public responded positively. Lewis published the two series of talks as Broadcast Talks in July 1942.
In fall 1942, Lewis broadcast his third series with the BBC. This time Lewis took as his subject “Christian Behaviour.” Instead of focusing on the stereotypical expectations of the public about Christian legalism, Lewis did not stress sin and vice. Instead he discussed the Christians virtues of faith, hope, charity, and forgiveness. He stressed that life in Christ, the living Lord, is a positive experience rather than a negative effort at self-improvement. This series was published separately as Christian Behaviour in 1943.
Lewis presented his fourth and final BBC series of seven talks beginning in fall 1943 and going into winter 1944. He named this series “Beyond Personality: The Christian View of God.” This series moved more profoundly into the mystery of the relationship between a believer and Christ. He discussed how Christ actually affects a person and changes them. He made clear that Christianity is not merely the gift wrapping that people may see in religious practices, but that the practices reflect a union that has taken place with God through the work of Christ Jesus. This last series was published as Beyond Personality in 1944. The public and the BBC asked for more talks, but Jack declined. When he had begun his talks, Britain stood alone against Hitler. By the end of his fourth talk, the Normandy invasion had changed everything and the outcome of the war seemed assured. In 1952 Lewis published the four series in a single volume he called Mere Christianity. The broadcasts and the publication of them on both sides of the Atlantic had produced a flood of letters with all manner of questions that Jack felt obliged to answer.
Yes, if the walls of The Kilns could talk, they would speak at some length about the writing of some of the most remarkable books of this century. The Screwtape Letters was written there, as was The Great Divorce, Miracles, much of Mere Christianity, and, I think, some of the Chronicles of Narnia. . . . It was in this gracious house that Jack Lewis died peacefully in 1963. (Walter Hooper, quoted in C. S. Lewis Remembered: Collected Reflections of Students, Friends and Colleagues [Harry Lee Poe and Rebecca Whitten Poe, general editors; Zondervan, 2006])
Located on the outskirts of Oxford, where Lewis taught for years, the Kilns was C. S. Lewis's home. He purchased the house in 1930 and remained there until his death in 1963. Originally built in 1922, on the site of a former brickworks (hence its name), it has been restored and now serves as a Christian study center. The address is The Kilns, Lewis Close, Risinghurst, Headington, Oxford, OX33 8JD, United Kingdom. If you'd like to visit, you can find information here.