Ever since I became a Christian I have thought that the best, perhaps the only, service I could do for my unbelieving neighbours was to explain and defend the belief that has been common to nearly all Christians at all times. I had more than one reason for thinking this. In the first place, the questions which divide Christians from one another often involve points of high theology or even of ecclesiastical history, which ought never to be treated except by real experts. I should have been out of my depth in such waters: more in need of help myself than able to help others. And secondly, I think we must admit that the discussion of these disputed points has no tendency at all to bring an outsider into the Christian fold. So long as we write and talk about them we are much more likely to deter him entering any Christian communion than to draw him into our own. Our divisions should never be discussed except in the presence of those who have already come to believe that there is one God and that Jesus Christ is His only Son. Finally, I got the impression that far more, and more talented, authors were already engaged in such controversial matters than in the defence of what Baxter calls ‘mere’ Christianity. That part of the line where I thought I could serve best was also the part that seemed to be thinnest. And to it I naturally went.
C.S. Lewis, Mere Christianity (1952); Anniversary ed. (2001) viii-ix.