One of my favorite books, Mere Christianity, is now free online to read and print (see here). If you have not read this classic you should.
If you want to get a sampling start by reading chapter 18 (my favorite) called "The Great Sin."
Here are the first two paragraphs:
I now come to that part of Christian morals where they differ most sharply from all other morals. There is one vice of which no man in the world is free; which every one in the world loathes when he sees it in someone else; and of which hardly any people, except Christians, ever imagine that they are guilty themselves. I have heard people admit that they are bad-tempered, or that they cannot keep their heads about girls or drink, or even that they are cowards. I do not think I have ever heard anyone who was not a Christian accuse himself of this vice. And at the same time I have very seldom met anyone, who was not a Christian, who showed the slightest mercy to it in others. There is no fault which makes a man more unpopular, and no fault which we are more unconscious of in ourselves. And the more we have it ourselves, the more we dislike it in others.
The vice I am talking of is Pride or Self-Conceit: and the virtue opposite to it, in Christian morals, is called Humility. You may remember, when I was talking about sexual morality, I warned you that the centre of Christian morals did not lie there. Well, now, we have come to the centre. According to Christian teachers, the essential vice, the utmost evil, is Pride. Unchastity anger, greed, drunkenness, and all that, are mere fleabites in comparison: it was through Pride that the devil became the devil: Pride leads to every other vice: it is the complete anti-God state of mind.