...and so, the Judge sentenced the murderer to go to the electric chair. But just then, the Judge's only Son piped up. "Please punish me instead! That way, he won't have to die." Out of his compassion for the criminal, the Judge agreed. The Judge's Son was executed, and the criminal went free. Tears pouring down his face, the killer vowed to be a new man from that day forward...
This parable is found nowhere in the Bible, but I'm pretty sure I've heard some version of it preached from the pulpit once or twice, as an analogy for what Jesus did for us on the Cross. Here's the problem: the story is ethically outrageous. How could punishing an innocent person instead of a guilty person possibly be just? In the story, the Son volunteers to die; it's not as though the judge just ordered the execution of some random person. But how could the guilt of punishment possibly be "transferred" from one person to another? The basic responsibility of the Judge to judge correctly is violated:
Acquitting the guilty and condemning the innocent—
the Lord detests them both. (Prov 17:15)
Admittedly, the story I began with was just meant to be an illustration, not the actuality. There are many different metaphors in the Bible to describe the Atonement, and most of them don't have to do with the justice system. The Bible talks about ransoming slaves, healing diseases, growing new life, being adopted as sons, and so on. But the criminal justice metaphor is one of the most common analogies in Western Christianity, so let's try to run with it for a moment.
Metaphors do not need to accord with the reality in every single way. It is even possible to illustrate righteous behavior by means of a person who, in the fictional story, behaves immorally. (For example, in Jesus' parable of the Dishonest Steward, the steward's clever way of enriching his master's debtors by fraud, to get special treatment from them later, is an analogy for how Christians should give away their "worldly" possessions to the poor in order to gain something more valuable. One is a sin and the other isn't, but Jesus' point is that they are similarly clever.)
However, in the (nonbiblical) story of the Judge's Son, the ethics of the story is so anomalous that it seems to render suspect any meaning which can reasonably be obtained from the story. The motivations of the characters don't make any sense, either from an altruistic or a selfish perspective, so it's unclear how we should react, other than with horror at the perversion of justice.
But now let's change the setting a little bit. We'll replace the criminal trial with a civil trial, and the death penalty with a fine:
A man breaks his neighbor's window. The neighbor sues, and the Judge orders the man to pay $200 to repair the broken window. However, the man is unable to pay, due to his poverty. So his friend kindly agrees to pay the fine instead. The friend pays $200 to the neighbor, and the windowbreaker goes free.
Suddenly, most of the ethical problems seem to evaporate. Most of us would have no problem with a Judge allowing this. In certain cases, we might feel like it is was a little unfair for a perpetrator to get off scott-free, because someone else paid the fine. But here, the windowbreaker couldn't pay. In light of the circumstances, the resolution of the case seems reasonable. What is the difference?
Part of it, presumably, is that we are more used to thinking of money as fungible than life. The concept of transferring debts is in accordance with our culture's common sense, while paying an innocent life for a guilty life is not (and rightly so)!
But I think the bigger issue here is the question of what the punishment is expected to accomplish. In the case of the fine, the main issue is that the window is broken! The fact that the windowbreaker is guilty comes in only secondarily. Given that the new window needs to be paid for, it seems fair to assign the liability to the man who—whether accidentally, or in a fit of rage—broke the first one. But if someone else wants to repair the window, that solves the problem: (1) The neighbor is compensated for the damage to his building, so he has no right to object, (2) The friend is allowed to do whatever he likes with his own money, and (3) the windowbreaker is enabled to pay the fine. No more problem!
Things are quite different in the case of a murderer, who pushes someone out the window and breaks their skull instead of the glass. The main problem is not the same.
One might be tempted to say that the main problem in the criminal case is that the victim is dead. But that isn't so! The death of the victim is the most tragic part of the situation, but it is not what the criminal trial is there to fix! Sentencing murders to death does not bring back their victims. Last I checked, not even a sentence of life-without-possibility-of-parole does that. No, in the criminal trial, punishing the criminal is the entire point of the proceedings (although there are multiple goals which this punishment might accomplish).
Another way to see this, is to compare to a situation where the victim dies accidentally. In this case, the death of the victim part is exactly the same. That they were murdered is the crucial difference. This fact is not located in the victim (who may not have known whether or not the fall was an accident), rather it is located in the mind and heart of the murderer. The murderer kills the body of the victim, but it is their own soul which they are doing violence to. If you murder someone, in the next moment you become the sort of person who would murder someone.
So then, this is the stain which the criminal punishment is supposed to fix. As Socrates says in the Gorgias, having wickedness in the soul is the worst thing that can possibly happen to you, and the guilty who are punished are benefited by it, since the punishment is a medicine for their wickedness.
It then becomes clear why it is impossible for an innocent person to justly take on the punishment of a murderer. Because it would not in fact fix the problem. In the criminal case, it is the murderer who is the broken window. Punishing the guilty party through the judicial system is our (usually very inadequate) way of trying to restore the window: to patch over the cracks with tape, or at least to sweep up the broken glass by taking away their power to hurt anyone else. Punishing an innocent person does no good at all. Unless...
Unless things were so arranged, so that the death of the innocent person actually did fix the broken window—or to drop the analogy, what if Jesus' death actually causes the stain in the murder's soul to be cleansed and purified? Suppose that, by accepting Jesus' death, the soul of the murderer is put to death (Romans 6:6-11) and then restored, so that the person who once hated his victim is now is full of love and compassion. In that case, justice would be done (but in a way invisible to the justice system, and perhaps even to society). The murderer would be simultaneously punished and forgiven by one and the same act of God.
How is that even possible? Well, I assume it has something to do with the divinity of Jesus; that when God assumed a human nature, this affected his relationship with every human being who ever lived. I suppose it has something to do with the Holy Spirit tampering with the neural network of our brain, after we give him permission to do so. I believe it has something to do with the omnipotent Father wanting to express his forgiveness through a tangible, observable event taking place in spacetime history.
But now we are asking a different question: does the Atonement make metaphysical sense? That is, is it the sort of thing which could happen in the real world, given the most basic structure of existence (whatever that is). The original question was whether the Atonement makes ethical sense. That is, supposing it to be possible, would it be desirable?
Assuming it is possible, it seems clear that offering the possibility of moral redemption to every person on Earth, no matter how wicked, is a very great benefit. One might still ask (if one is inclined to second-guess the Creator) whether we really need such a desperate remedy, and why God did not provide forgiveness in some less bloody way. This does not change the fact that it works.
At the Cross, we see God's solidarity with human beings. He suffers with the innocent, and for the guilty (and we have all been both at times). The Cross shows up the depths of human depravity, and reveals that the primary victim of our sin has always been God. But it is also the triumph of God's mercy, because it shows us that no matter what suffering we cause to ourselves and others, God is there accepting the pain, refusing to retaliate and offering continual forgiveness. I cannot imagine any more graphic way for God to show this, than the way he did.