The Difficulty of Resisting Temptation

No man knows how bad he is till he has tried very hard to be good. A silly idea is current that good people do not know what temptation means.
This is an obvious lie. Only those who try to resist temptation know how strong it is. After all, you find out the strength of the German army by fighting it, not by giving in. A man who gives in to temptation after five minutes simply does not know what it would have been like an hour later.
That is why bad people, in one sense, know very little about badness. They have lived a sheltered life by always giving in. We never find out the strength of the evil impulse inside us until we try to fight it.

- C. S. Lewis, Mere Christianity