Explaining the necessity of the atonement

From Tim Keller, notes from talks given at Gordon Conwell Theological Seminary on preaching, here

Why is there need for atonement? Why does Christianity say that Jesus had to die in order for us to be re-united with God? Why can't God just forgive us? The answer is that no one can "just" forgive any serious wrong. If someone has betrayed you deeply and caused great harm - how do you forgive them? Forgiveness means refusing vengeful actions when you deeply want to make them pay for what they did. It means refusing to 'run them down' to others when you deeply want to slice up and ruin their reputation. It means even refusing thoughts of ill-will and rather turning your thoughts to pity and hope for their change. And as time goes on - if you stay the course, the anger will go away and the forgiveness is complete.

As anyone knows who has ever tried it - this is extremely painful, costly, and agonizing. If you do not forgive, you become hard and angry yourself, and a cycle of revenge and conflict goes on and on - so evil triumphs. On the other hand, if you do do the way of forgiveness, you will experience a great deal of pain and suffering yourself. There is no middle ground. Either you can make the perpetrator pay down the debt you feel (as you take it out of his hide in vengeance!) in which case evil wins - or you will absorb the debt yourself. It is the same in the economic realm as in the psychological realm. If someone knocks over your $100 lamp and says, "I'm so sorry" and you say "forget it!" you have forgiven them. But the $100 debt does not vanish into thin air. Either you make them pay it or absorb it yourself (by buying a new lamp or going without light in that corner.) So we see this principle - that when a serious wrong is committed, there is a "debt" that cannot be ignored or dismissed but must be dealt with, and that it must be dealt with through suffering.

Now, if we see this principle at our human level - that only way to defeat evil is through forgiveness that entails suffering - why are we surprised when we hear God telling us it is the same with him?... If when we are wronged we sense a debt cannot be just willed away, that must be paid for with suffering - how much more is God aware of the enormous debt of human beings' sin against one another and against the creation and against God himself. Either there must be judgment so that we suffer, or there must be forgiveness so God must suffer! There is no middle way. He cannot "just forgive" either. On the cross, God paid the debt himself. There we see at the spiritual and cosmic level what we know unavoidably at the psychological and relational level.