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 himself 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
Compiled in A Year with C.S. Lewis