" I would much rather say that every time you make a choice you are turning the central part of you, the part of you that chooses , into something a little different from what it was before. And taking your life as a whole, with all its innumerable choices, all your life long you are slowly turning this central thing either into a heavenly creature or into a hellish creature: either into a creature that is in harmony with God, and with other creatures, and with itself, or else into one that is in a state of war and hatred with God, and with its fellow-creatures, and with itself. "
Mere Christianity gives an argument for the truth of Christianity, and tells what Christans believe. Lewis explains how the Christan World View is the only belief system to adequately answer the questions men ask about their own existence.
He begins by giving the basic idea that all men have a built-in moral code that they demand others to follow, but that they themselves ignore. He goes on to say there is a part inside each person that makes them feel guilt at ignoring this standard. (A man will acknowledge that he has not been fair, but he will have reasons why this was acceptable in his case.) Our sense of justice is a proof that God exists, not because we expect other people to 'keep to the code', but because we feel the need to excuse ourselves when we do not.
Lewis then looks at the practice of the Christan life. What does a Christian life look like when followed to its logical conclusion? The answer is both convicting and encouraging. However, it is unlikely that a non-Christian or atheist would be able to grasp many of the concepts. As Lewis says, the more badness a man has the less likely he is to see it.
Finally, I do respect Mr. Lewis very much and I am thankful for the gift God gave him in encouraging other Christians in their faith. I do have a different understanding from Lewis' belief that anyone who is sincere will take part in heaven. At this point I am still of the opinion that you must sincerely accept Christ, and that it is Christ, not sincerity that gets us into heaven. I may be misunderstanding Lewis or simply wrong in my own belief, but there it is.
Lewis is a wonderful writer. His style makes you want to keep reading no matter how heavy the content. He makes you think and develop your reason. His writing is clear and understandable to most people, but insightful and compelling at the same time.