Though Christian charity sounds a very cold thing to people whose heads are full of sentimentality, and though it is quite distinct from affection, yet it leads to affection. The difference between a Christian and a worldly man is not that the worldly man has only affections or ‘likings’ and the Christian has only ‘charity’.
The worldly man treats certain people kindly because he ‘likes’ them: the Christian, trying to treat every one kindly, finds himself liking more and more people as he goes on—including people he could not even have imagined him- self liking at the beginning.
This 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 railway line or bridgehead from which the enemy may launch an attack otherwise impossible.
From Mere Christianity, by C S Lewis