The worldly man treats certain people kindly because he "likes" them: the Christian, trying to treat every one kindly [even those he does not like], finds himself liking more and more people as he goes on - including people he could not even have imagined himself liking at the beginning.
The same spiritual law works terribly in the opposite direction. The Germans, perhaps, at first ill-treated the Jews because they hated them: afterwards they hated them much more because they had ill-treated them. The more cruel you are, the more you will hate; and the more you hate, the more cruel you will become - and so on in a vicious circle for ever.
Good and evil both increase at compound interest. That is why the little decisions you and I make every day are of such infinite importance. The smallest good act today is the capture of a strategic point from which, a few months later, you may be able to go on to victories you never dreamed of. An apparently trivial indulgence in lust or anger today is the loss of a ridge or a railway line or bridgehead from which the enemy may launch an attack otherwise impossible.
- C.S. Lewis, Mere Christianity, 111.