What Goodreads has to say:
Mere Christianity is C.S. Lewis's forceful and accessible doctrine of Christian belief. First heard as informal radio broadcasts and then published as three separate books - The Case for Christianity, Christian Behavior, and Beyond Personality - Mere Christianity brings together what Lewis saw as the fundamental truths of the religion. Rejecting the boundaries that divide Christianity's many denominations, C.S. Lewis finds a common ground on which all those who have Christian faith can stand together, proving that "at the centre of each there is something, or a Someone, who against all divergences of belief, all differences of temperament, all memories of mutual persecution, speaks the same voice."
What I have to say:
Everyone is born with an innate sense of right and wrong. This sense is the thing that prompts us to help someone in need, apologize to someone we have wronged, treat others with courtesy, and so on. All humans share this sense; we may have different cultures and belief systems, but everyone agrees that it is wrong to do certain things (such as stealing, committing murder, double-crossing someone who has been good to you) and that it is right to do other things (such as telling the truth, keeping your promises, and thinking of others before yourself).
The fact that we all have this same sense of right and wrong shows that there must be something out there from which we received this sense - something or someone greater than us who wants us to behave in a certain way. But we have all failed to live up to this standard that has been set for us; we sometimes put ourselves before others, and we sometimes are dishonest or lose our patience with someone who loves us, and so we must be at odds with this greater something or someone who wanted us to behave in this way.
With this basic argument, C.S. Lewis opens Mere Christianity, a book adapted from the broadcast talks which he gave on the radio during WWII. Logical, straightforward, and convincing, Mere Christianity starts at the very beginning, neither assuming nor even suggesting the presence of a God, and gradually works its way up to Christianity through flawless reasoning mixed with scatterings of humor and wit. From the simple fact that all human beings share a sense of right and wrong, Lewis deduces that there must exist some greater power who has endowed us with this sense and wants us to act in a certain way. This power could be a presence who is beyond good and evil and constitutes the universe (this is the Pantheistic view) or a distinct being who loves good and hates evil (this is the Theistic view). However, the fact that this higher power has endowed us with a distinct sense of right and wrong tells us that he does believe in a right and wrong, and is therefore not "beyond" good and evil. From here, after some further reasoning, Lewis eventually reaches the God of Christianity. This God is a distinct entity who believes in right and wrong and has commanded men to act according to this system. But, because men have made themselves God's enemies through failing to live up to His standard, He has provided a way for us to be reconciled to him: namely, He has sent His son to atone for our sins. Christ, who is the son of God, has suffered and surrendered for us, so that we, by sharing in his sacrifice, can become forgiven and at one with God.
I've never read a book that made a more convincing case for Christianity. Lewis' argument is so methodical and logical - it's as if he reasons his way to Christianity. Reading this book, I realized for the first time just how much sense Christianity makes. The second half of the book lays out the teachings and values of Christianity, opening them up in new ways and making sense of things that many people (myself included) often have trouble understanding. However, there is a section where he talks about the Trinity and the Nature of God, and since Lewis was an Anglican, his beliefs on this subject differ from mine. But I still give him full credit for doing the best he could with what he had, and the rest of the book should align almost entirely with the beliefs of any Christian from any denomination.
I've also found that this book lays the groundwork for an understanding of all of Lewis' other writings: it's like a jumping off place where he articulates his ideas in detail, and then his other works connect to these ideas and play off of them in new ways. Many of the ideas he articulates elsewhere are given fuller form in this book, and I've found that it's a helpful starting place for tracing his ideas.
This book is probably Lewis' most quotable work; his single liners blew me away. Here are some of my favorites:
"True humility is not thinking less of yourself; it is thinking of yourself less."
"Nothing you have not given away will ever really be yours."
"Faith, in the sense in which I am here using the word, is the art of holding on to things your reason has once accepted, in spite of your changing moods."
"Do not waste time bothering whether you ‘love’ your neighbor; act as if you did. As soon as we do this we find one of the great secrets. When you are behaving as if you loved someone, you will presently come to love him."
"We see only the results which a man's choices make out of his raw material. But God does not judge him on the raw material at all, but on what he has done with it."
"If I find in myself desires which nothing in this world can satisfy, the only logical explanation is that I was made for another world."
If those didn't persuade you to read Mere Christianity, than nothing I can say will do so. I think the last one alone would persuade me. So if you want to be spiritually edified and enlightened, and if you want to spend some quality time with C.S. Lewis, pick up Mere Christianity and give it a try. You won't regret it.
If you still need convincing (and even if you don't) here's a picture of Jack (alias C. S. Lewis) for you: