I haven't read much of atheist Christopher Hitchens - only enough to recognize his brilliance - and I certainly don't plan to read his book God Is Not Great, but recently I heard only a portion of a debate in which he argues that the doctrine of vicarious atonement is an immoral and highly dangerous one - this is the teaching that God the Son joins a human nature in order to offer himself to the Father on our behalf, that we may attain the forgiveness of our sins.
Hitchens argues that to take a guilty man's place of punishment, for example, to go to prison in his stead or to submit to execution on his behalf, is repugnant to the very idea of justice; for the criminal alone can pay his debt, not the innocent man. If I who am innocent decide to go to prison or the scaffold instead of the killer, then I cooperate with evil and perpetuate an unjust state of affairs - the killer remains at large, while an innocent man is in prison or worse, put to death.
But that is not an analogous case that accurately illustrates the doctrine of Christ's redemption.
First, if someone is indebted to me, it is not repugnant to the very idea of justice for me to forgive that debt. That is, rather, an act of mercy, and mercy transcends justice. So, if someone owes me a debt so great that he cannot repay it fully, I can remit the debt, cancel it, and bear the loss, if I so choose, especially if my purpose in forgiving that debt is to communicate the love I have for my debtor.
God can cancel our debt if He so chooses. And we, the human race as a whole, have a debt to Him that we cannot fully repay. He has chosen to remit that debt. But He also chose to remit it in a way that visibly manifests His love for us who are, out of His ninety-nine sheep, the single sheep that has gone astray (Lk 15, 4-7). But He remits this debt in a way that fulfils both the requirements of justice and at the same time reveals His infinite mercy.
Secondly, it is not unjust for the human race as a whole to seek out God's forgiveness for its own sin. The problem is that the human race as a whole can do nothing about this debt. It cannot pay the debt and make satisfaction for sin, because sin against God is of infinite gravity. But it would be fitting for one member of the human race to come forward on behalf of all of humanity and offer something in reparation, as a lawyer steps forward to represent a corporation, or an ambassador represents a country.
So God provides the solution. He joins a human nature to Himself: Jesus is two natures (human and divine), but one Person (the Person of the Son), and he (Jesus) acts on our behalf, as an ambassador acts on behalf of a nation. He is our voice. Now that is fitting; for he is truly man, a member of the human race.
But he is also fully God, and so what he offers, namely his own life, has a value that is not limited. Unlike anything we can offer, the value of Christ's offering is commensurate to the debt of sin, which is of infinite gravity. There is nothing repugnant to justice here. It is an act of mercy, a gift; his sacrificial act manifests the divine love for man, and it cancels the debt of sin so that the life of God (divine grace) can once again flow through the veins of humanity, justifying each person who freely chooses to cooperate with that grace.
Hitchens' argument does not stand because his analogy is a false one. For me to take the place of a killer and go to prison in his stead is indeed repugnant to justice because the criminal has committed an injustice against the civil community as a whole (as well as a sin against God), not against me personally. His sin is not for me to forgive, for it is not against me personally. My going to prison does not balance the scales of justice, because it leaves intact an unjust state of affairs - the killer is still on the loose. A more accurate analogy is the cancellation of a debt owed to me personally by one individual person, or an individual family, a debt that cannot be fully remitted. To continue the analogy, a member of that family steps forward to offer a sacrifice of reparation, on behalf of that family, in order to remit the debt. Doing so, of course, is impossible, and so we are in an impossible situation - unless I, who have an unlimited source of funds, can somehow become a member of that family and make that offering myself, so that my mercy would be visible to each member of that family.