Mere Christianity* was originally four series of radio broadcasts on the BBC between 1941 and 1944. Rev. James Welch, Director of Religion for the BBC, had read C. S. Lewis's book The Problem of Pain (1940) and, without ever hearing Lewis's voice, approached him about doing a radio series.**
A format known as the "radio talk" came into its own during World War II Britain. There were radio talk programs on victory gardening, cooking with home-grown ingredients, and medical care. Radio talks were low budget and easy to produce. Most were broadcast live because the materials needed to make recordings -- primarily metal -- were in short supply. Consequently, only three of the broadcasts, which came to comprise Mere Christianity, were recorded.
During the war, programs had to be cleared by a censor before airing. In addition, the timing had to be exact, as the BBC feared that a German transmission could break in if there were an extended pause or silence. This forced Lewis to write scripts for each talk that filled a precise amount of time (usually 10 or 15 minutes), no more, no less. Moreover, a radio listener cannot flip back a page to re-read a point missed the first time, the way readers of a book can. This meant Lewis's message had to be simple and clear so the audience would grasp it in a single passing.
The impact of these broadcasts and the subsequent book, Mere Christianity, cannot be underestimated. The radio scripts developed Lewis's ability to write precisely and for the ordinary person. He formed a reputation as a spokesman for Christianity that generated volumes of mail over the years to come -- letters he felt obligated to answer personally. Most of all, reading this book has changed the lives of countless people.
* What we know today as Mere Christianity was originally published in three separate volumes: The Case for Christianity (1943); Christian Behavior (1943); and Beyond Personality (1945).
** Information in this summary is drawn from Justin Phillips, C. S. Lewis in a Time of War: The World War II Broadcasts That Riveted a Nation and Became the Classic Mere Christianity (2002).
Phillips, Justin, C. S. Lewis in a Time of War: The World War II Broadcasts That Riveted a Nation and Became the Classic Mere Christianity (2002) - an account of Lewis's famous radio talks from the perspective of the BBC; also describes the role of radio on the British home front during WWII.
Vaus, Will, Mere Theology: A Guide to the Thought of C. S. Lewis (2004) - expands many of the Christian doctrines introduced in Mere Christianity to include related thoughts expressed in his other writings.
http://www.scholia.net/ - Focus Guide for the Study of Mere Christianity prepared by a Sunday School Teacher at Our Redeemer Lutheran Church of Emmett, Idaho (2002) - a study guide for those desiring a Lutheran perspective.
http://www.gordy-stith.com/ - another online set of discussion questions for Mere Christianity