Here is another passage from Mere Christianity, this time from the chapter "The Rival Conceptions of God."
I have been asked to tell you what Christians believe, and I am going to begin by telling you one thing that Christians do not need to believe. If you are a Christian you do not have to believe that all the other religions are simply wrong all through. If you are an atheist you do have to believe that the main point in all the religions of the whole word is simply one huge mistake. If you are a Christian, you are free to think that all these religions, even the queerest one, contain at least some hint of the truth. When I was an atheist I had to try to persuade myself that most of the human race have always been wrong about the question that mattered to them most; when I became a Christian I was able to take a more liberal view. But, of course, being a Christian does mean thinking that where Christianity differs from other religions, Christianity is right and they are wrong. As in arithmetic - there is only one right answer to a sum, and all other answers are wrong: but some of the wrong answers are much nearer being right than others.