CS Lewis’s Mere Christianity: A Biography by George Marsden Princeton, £18.95 A simple-sounding quest lies at the heart of CS Lewis’s Mere Christianity: the search for “the belief that has been common to nearly all Christians at all times”. The word “mere” should not be confused with “merely”: it denotes “a standard of plain, central Christianity ... which puts the controversies of the moment in their proper perspective”. The essentials are what counted for Lewis – the ideas that had been tested over time rather than the latest fad or devotional fault line. This all sounds rather jolly, and doubtless accounts for the book’s enduring, cross-denominational appeal, but what does “mere Christianity” really mean? What role do the deposit of tradition or important doctrinal differences have to play? Lewis wrote of “a hall out of which doors open into several rooms”. It is in the rooms, which represent individual denominations, “that there are fires and chairs and meals”. You are at liberty to make your selection but you must “be kind to those who have chosen different doors”. After all, “if they are wrong they need your prayers all the more; and if they are your enemies then you are under orders to pray for them”. All told, perhaps a better title for the book might have been Cosy Christianity. George Marsden provides a splendid account of the book’s evolution (it started out as a series of radio broadcasts) and its reception. We learn what the critics had to say, how the book influenced many individual spiritual trajectories and how it continues to do very well in polls of the most influential Christian writings. Was Lewis a serious Christian thinker with the common touch, or was he a brilliant rhetorician who lacked theological depth? Mere Christianity is a good place to look for answers. Alister McGrath describes the book as “an informal handshake to begin a more formal acquaintance and conversation”, but there is something much too vague, simplistic and potentially debilitating about Lewis’s understanding of a shared, “plain” Christianity.