Two quotes from his book:
First, All I have got is a Something which is directing the universe and which appears in me as a law urging me to do right and making me feel responsible and uncomfortable when I do wrong.
Second, There are those who hold the view that the lowest forms to Maqn were not due to chance but to the 'striving' or 'purposiveness' of a Life-Force. When people say this we must ask them whether by Life-Force they mean something with a mind or not. If they do, then "a mind bringing live into existence and leading it to 'perfection' is really a God, ant their view is thus identical with the Religious. If they do not, they what is the sense in saying that something without a mind 'strives' or has 'purpose'?"
Lewis beats you up with his logic. You cannot escape the progressive path his mind takes when reasoning out one of the biggest questions about which men have argued since there were men to argue.
He is moving step by step to his conclusion, but let's stay with his words for awhile and see if he moves us with him, along this path where a Law of Right and Wrong seems to clobber us ... where this law was not our own invention like stopping for a red light, but comes from outside ourselves.
If Man created morality, as many argue, why would we so completely ignore it or even destroy it If from outside ourselves, from where does it originate and for what purpose is there such a law at all?
As Chapter 5 of "Mere Christianity" begins, Lewis admits there must be some doubters who refuse to follow along with him because they smell a skunk in the grass.
"I expect when I reached this point, some of you felt a certain annoyance. You may even have thought that I had played a trick on you -- that I had been carefully wrapping up to look like philosphy what turns out to be one more religious jaw."
In his defense, he reminds us that he is nowhere near defining a God, still less the God of Christianity.
"We have only got so far as a Somebody or a Something behind the Moral Law. We are not taking anything from the Bible or the Churches, we are trying to see what we can find about this Something from our own steam. We have two bits of evidence about that Somebody. One is the universe that He has made. The other is this Moral Law which he put into our minds."
Lewis sees this Moral Law as more revealing and this is perhaps why he spends so much time (Four and a half chapters of his book) exploring the nature of this Law. It is important because it is inside all of us (even inside the bad guys) so it is a huge clue about the Somebody who put it in our minds.
Somebody that puts such a constraint around everyone living on this planet must be a lot bigger than the planet itself. This Somebody is outside of us, but is intensely interested in us. And not just interested like watching a figure skater on ice ... not like watching what hamsters do when a wheel is placed in their cages ... but much more like watching us when we have before us options to do a right thing or a wrong thing.
"From this second bit of evidence, we can conclude that the Being behind the universe is intensely interested in right conduct ... in fair play, in unselfishness, courage, good faith, honesty and truthfulness."
Lewis goes on to clarify that this Being is not going easy on us. The Moral Law is "tough as nails" -- it demands sacrifice, pain, danger, difficulty. This Power behind the Law has told us nothing about forgiveness, but only that we must do the Right Thing. If this Power wants us to do the right thing, we can assume that it does not like it when we do the wrong thing. And if we are honest, even the best of us do a lot of wrong things. You could quickly surmise then that this Power does not like much about us.
Now you are looking into the eyes of a Power that does not have much to like about us.This Power has every reason to be upset with us, angry with us, downright MAD at us.
But if this Power has taken all the time an effort over eons to create life and gradually form it into us, and this Power has given us a Law to follow, then it could follow that this Power sees a greater potential in us that we can see in ourselves. Out of that glimmer, we have hope.
Footnote: this battle rages on, long after Professor Lewis made his case for "Mere Christianity. The image above is from a site where an antheist does his best to outwit Katie, one of the most incredible 17-year old girls in all Christendom. The site author tears at God and Jesus but she battles back until all he has left in him is rants. She's praying for him and I hope we all do.
This ends Book One of Mere Christianity. Professor Lewis moves on next to "What Christians Believe."