From "Mere Christianity" by C. S. Lewis.,
In the long run God is no one but himself and what he does is like nothing else. You could hardly expect it to be. What, then, is the difference which he has made to the whole human mass? It is just this; that the business of becoming a son of God, of being turned from a created thing into a begotten thing, of passing over from the temporary biological life into timeless spiritual life, has been done for us. Humanity is already saved in principle. We individuals have to appropriate that salvation. But the really tough work, the bit we could not have done for ourselves, has been done for us. We have not got to try to climb up into spiritual life by our own efforts, it has already come down into the human race. If we will only lay ourselves open to the one man in whom it was fully present and who, in spite of being God, is also a real man, he will do it in us and for us. Remember what I said about “good infection.” One of our own race has this new life: if we get close to him we shall catch it from him. Of course, you can express this in all sorts of different ways. You can say that Christ died for our sins. You may say that the Father has forgiven us because Christ has done for us what we ought to have done. You may say that we are washed in the blood of the Lamb. You may say that Christ has defeated death. They are all true.
But, you will ask, does this much mend matters? Is not this still injustice, though now the other way round? Where, at the first glance, we accused God of undue favour to his chosen, we are now tempted to accuse him of undue disfavour (the attempt to keep up both charges at the same time had better be dropped). And certainly, we have here come to a principle very deep-rooted in Christianity: what may be called the principle of vicariousness. The sinless man suffers for the sinful and, in their degree, all good men for all bad men. And this vicariousness, no less than death and rebirth or selectiveness, is also a characteristic of nature. Self-sufficiency, living on one’s own resources, is a thing impossible in her realm. Everything is indebted to everything else, sacrificed to everything else, dependent on everything else. And here too we must recognise that the principle is in itself neither good nor bad. The cat lives on the mouse in a way I think bad, the bees and the flowers live on one another in a more pleasing manner. The parasite lives on its host but so also the unborn child on its mother. In social life, without vicariousness there would be no exploitation or oppression but also no kindness or gratitude. It is a fountain both of love and hatred, both of misery and happiness. When we have understood this we shall no longer think that the depraved examples of vicariousness in nature forbid us to suppose that the principle itself is of divine origin.