What Lewis tries to achieve with “Mere Christianity” is to bring non-believers into the “hall” of Christianity. He specifies that that is his only intention; choosing which door to enter(the denomination) is up to the person himself while he is waiting in the hall.
Thus the first chapter of this book tries to convince people about the existence of a Law, or more specifically, the presence of the Law of Human nature. This is a standard behavior which everyone agrees on being morally correct, a rule that a person can choose either to obey or disobey. Lewis lays proof of its existence using extraordinary logic. One of my favorite examples he uses is when he writes that even if “you find a man who says he does not believe in a real Right and Wrong, you will find the same man going back on this a moment later. He may break his promise to you but if you try breaking one to him he will be complaining ‘It’s not fair’ before you can say Jack Robinson.” Just like it was mentioned in one of the lectures in our class where a flow chart was shown, if someone claims to not believe in the existence of a moral right, one can do anything to them, and they would have no right to complain (However, this in turn would be morally incorrect).
Unfortunately, no one is really keeping this law totally. People offer excuses, a special reason for not being to meet up the ‘how it ought to be’ behavior. But this is just another evidence for the existence of the Law of human nature. The guilt one feels when one breaks the rules, shows that there is a law imprinted on us. Thus I think that just as no one is able to obey it fully, no one can truly escape it.
In chapter two, Lewis brilliantly refutes certain opposing statements. One of them objects to the Law’s existence by saying it might be a “herd instinct.” Lewis pin points that an instinct would merely be a strong desire to act in a certain way and that if there were two instincts to choose from, there would still be a measuring stick (the law) that decides which one is to be acted out. “There are no such things as good and bad impulses…” he writes, and complements that every single one “ ..is right at one time and wrong at another. The Moral Law is not any one instinct or a set of instincts: it is something which makes a kind of tune (the tune we call goodness or right conduct) by directing the instincts.” To all this I add, if morality were to be an "herd instinct" it would be something done by the majority as the word "herd" indicates, however, keeping morality is the exact opposite of what everyone is actually doing (since everyone sins); thus morality cannot be an "herd instinct" anyways.
The second objection proposes for the moral law to be something invented by education. However, this too is proved wrong by the acceptance that moral law can be taught however, it is not simply a “human convention,” but a “real truth” (just like logic). This can be seen in different cultures and different times. One can notice that each moral ideal is not much different from each other. For example in any of them, “selfishness has never been admired”.
Chapter three goes on, in general, about how decent behavior is simply decent behavior. It is not something created to be necessarily useful or convenient (in fact, if one thinks about it, doing the right thing is sometimes much more inconvenient than doing something wrong) but because it is the way it ought to be and ought to be followed.
This takes us to the fourth chapter: What lies behind the law. Lewis firmly states that Science cannot decide if ‘Something behind’ is or is not. It is unable to do so because science consists on experimenting, and finding out whether there exists ‘something behind’ is an issue beyond facts we are able to observe and experiment.
Overall, the whole reading points at and argues there is a Moral Law that serves as a standard for ‘good’ behavior, and that therefore there must be something, or someone, directing the universe. With easy examples, and very reasonable deductions, Lewis’s “Mere Christianity” is a book that exists to convince, and which has convinced me twice of my own faith.