Another problem that I have with the PST view of the atonement is that it effectively eliminates true forgiveness. In other words, true forgiveness does not require payment or punishment. If the penalty for man's sin is extracted from Jesus Christ, then it is not forgiven, its paid for by Christ. If, I owe someone a $20 and my friend pays that debt for me, the debt is not forgiven--its paid. On the other hand, if the person I owe the money to forgives me the debt, then no one has to pay it.
In the book, Pierced for Our Transgressions: Rediscovering the Glory of Penal Substitution by Steve Jeffrey, Michael Ovey and Andrew Sach, the authors deal with this objection (pp. 263-265). Their response is as follows: The reason why Penal Substitution does not deny that God forgives sin is precisely because it is God himself , in the person of his Son, who pays the debt we owe. (p. 264).
I fail to see how that explanation resolves the problem. No matter who pays it, it is paid and not forgiven.
A person to whom a debt is owed cannot wipe it out by "repaying" it himself. If you lend me $20 that you have in your wallet and I fail to repay it and then you go get another $20 from the ATM and replenish the cash in your wallet, you have not repaid my debt at all but you have just simply moved the money around and I still owe you $20. You are not forgiving me because you have another $20 in your wallet, you are forgiving me the debt because either I can't pay it or I don't want to pay it.
On the other hand, if my friend pays the $20 back to you on my behalf, there is nothing for you to forgive but I am still in debt—this time to my friend. The only way you could truly forgive my debt is by allowing it to remain unpaid or accepting something less than full payment as sufficient (in that case, you would be forgiving the remainder of the debt).
To use a non-monetary analogy, lets say that I slap your face. You can forgive me without extracting anything from me or you can repay me by slapping my face. If you choose to slap my face, you are not forgiving me, you are extracting justice. You could require some type of “partial payment”, however, and still be able to practice real forgiveness. In other words, you may choose to forgive me IF I say I am sorry or IF I promise not to do it again, etc.
True forgiveness, on the other hand, is seen in Luke 7:36-50, where Jesus forgives the woman who washed his feet with her hair and tears. He illustrates this real forgiveness in verses 41-42 when he says: There was a certain creditor which had two debtors: the one owed five hundred pence, and the other fifty. And when they had nothing to pay, he frankly forgave them both. Tell me therefore, which of them will love him most? (KJV)
The creditor forgave the debt without requiring any payment. Jesus seems to indicate that his forgiveness of the sinner woman in this passage is along the same lines.
In the so-called "Lord's Prayer" in Matthew 6:12-15, we read: And forgive us our debts, as we also have forgiven our debtors. And do not lead us into temptation, but deliver us from evil. For if you forgive others for their transgressions, your heavenly Father will also forgive you. But if you do not forgive others, then your Father will not forgive your transgressions. (NASB)
So, when we forgive those who trespass against us, are we doing so without requiring payment in full from them for their trespass? Yes, otherwise its not really forgiveness.
To summarize: My point is that while there can be stipulations on forgiveness, to require full equivalent payment or punishment (which is precisely what PST teaches) is not forgiveness. PST teaches that Jesus bore exactly the amount of punishment on the cross that each individual sinner (or each one of the elect if you are Calvinist) deserved and, therefore, he made a full payment for the sinner. If that is the case, then the sinner is not being forgiven. There is nothing to forgive. The sin has been wiped out (i.e., expiated) by the atonement of Christ.