It is true that in Aquinas' view of the Atonement, Jesus is not punished by the Father for man's sin, as he is in the PST. Rather, Jesus' life and death are offered up as a sacrifice to the Father and the Father accepts that sacrifice as sufficient satisfaction for man's sin. Aquinas' view does escape some problems faced by adherents of the PST, namely, how one member of the Trinity could be punished by another member and how one member of the Trinity could endure spiritual death (i.e., separation from God). These are very serious theological problems which any defender of the PST must face. However, does Aquinas' view escape the problem of how an innocent person can suffer in place of the guilty? I don't think it does. While Jesus is not technically being punished in Aquinas' view, he is still suffering and he is doing it in lieu of the punishment that sinners deserve. The Father accepts his life and death as satisfactory to eliminate the punishment of believers.
An important question at this point is: "How can God accept the death of an innocent person as satisfactory payment for the penalty owed by the guilty?" It seems inescapable to me that some form of substitution is taking place here. The death of Jesus is being accepted by the Father as a satisfactory substitute for the penalty owed by sinners. Joy attempts to answer the problem:
While there is nothing at all contrary to justice in one man voluntarily taking upon himself a certain penalty in order to satisfy for another’s sin, it would be manifestly unjust for a judge to inflict a punishment upon an innocent man no matter how willing he is to accept it. The point at issue is the agent of the act. As an act of vindictive justice, punishment belongs to the judge, while satisfaction belongs to the penitent as an act of the virtue of penance. Hence, whereas it is an act of virtue to take a penalty upon oneself in order to satisfy for a friend, it would be nonetheless vicious for a judge to inflict a penalty upon the innocent in place of the guilty, even if the innocent party were willing to accept it. Since satisfaction is made by the voluntary assumption of penal works, there can be no question of injustice on the part of the judge, who merely accepts the voluntary offering of the innocent friend as sufficient and therefore inflicts no punishment on the guilty (nor indeed on anyone). Penal substitution theories go astray precisely when they regard Christ’s death as a punishment actively inflicted on him by God the Father, rather than as a voluntary act of satisfaction on the part of Christ, which is permitted and accepted by the Father (pp. 50-51).
I see several problems with this explanation. First, it is not at all clear from Scripture that the Father is not involved in the death of Jesus. The NT consistently states that Jesus came to earth and died in order to fulfill the Father's plan (Rom. 8:32; Acts 2:23) and that he was obedient to the Father's plan (Phil. 2:8; Heb. 5:8; Jn. 10:18) even though he struggled with it in Gethsemane (Matt. 26:42; Mk. 14:36; Lk. 22:42; Jn. 12:27). So, while Aquinas may be correct in saying that the Father is not the direct agent in Jesus' death in the sense that the Father is himself punishing the Son; nevertheless, it does not seem to lessen the Father's culpability in the death by saying that he allowed others to do the "dirty work." If one plans a crime and then allows others to carry out the plan, one is still guilty of the crime. If one says that the executioners of Jesus meant it for evil but God meant it for good (as in the case of Joseph's brothers in Gen. 50:20), it still involves moral problems. At the least, it seems that God is guilty of moral relativism, i.e., the end justifies the means.
Second, why is the death of Jesus needed to satisfy God? It seems that the Scriptural answer to this is that death (specifically a bloody death) is necessary to take away sin. The author of Hebrews states it this way: without the shedding of blood there is no forgiveness of sins (9:22). Paul says that the wages of sin is death (Rom. 6:23). The clear implication in Scripture is that death is the penalty for sin (cf. Gen. 2:17). So, it seems that the reason Jesus died was in order to pay the penalty for sin; otherwise his death was unnecessary. If God could forgive man's sin without a bloody sacrificial death, then it seems cruel and sadistic for God to have come up with this plan for redemption. One must ask at this point, what kind of person (human or divine) takes pleasure (cf. Isa. 53:10) in the torture and death of another person? It definitely seems to be unworthy of a perfectly holy God.
Third, it is still not clear how God can justly accept the suffering of one in place of another. Anselm and Aquinas make a distinction between punishment and satisfaction. Joy uses the illustration of a man who offends his wife being able 1) to make satisfaction for the offense by giving her flowers (and being remorseful) or 2) to be punished for the offense by being forced to sleep on the couch. He says that either one serves justice. While one can see this to be the case in the example given, how could another person step in and offer flowers to the wife in place of her husband and the wife accept the offering as satisfaction for what her husband did? It doesn't make sense. Joy attempts an explanation:
On account of the union of charity between two friends, the satisfaction made by one on the other’s behalf really becomes in a way also the act of the other, for charity regards a friend as another self and suffers with the suffering friend: “and thus punishment is not lacking to him, as long as he suffers with his suffering friend; and so much the more fully as he himself is the cause of his suffering" (Aquinas, Summa Contra Gentiles III, cap. 158, n. 7 cited in Joy, pp. 49-50).
So, Aquinas is saying that a friend of the offender can offer a gift to the offended party on behalf of the offender and the offended party is justified in accepting it as if it came from the offender himself. He says that the offender is also suffering along with his friend because he realizes that his offense is the cause for the sacrifice that his friend is making. I am not sure what type of justice this illustrates but it is not retributive justice. I think the Bible clearly teaches retributive justice and whatever view of the atonement one holds must align with this view of justice.
Aquinas maintains that God's wrath against sin is "satisfied," or "propitiated" by the death of Jesus. Joy writes:
[F]rom Thomas's point of view . . .the wrath of God is appeased by Christ’s sacrifice. Wrath that is “appeased” or “propitiated” is precisely not poured out, but rather assuaged. A wrathful person is appeased when his anger is calmed, not when it is unleashed in all its fury, whether upon the guilty or the innocent (p. 52).Let me propose an illustration of this type of "justice." A man commits a rape against my child. While he is in jail, before the trial, he is beaten to a pulp by other inmates and is paralyzed. I meet with the criminal and he is extremely remorseful for his action of raping my child. Seeing the suffering that he has endured, and that he will be paralyzed for the rest of his life, I forgive the man and drop the charges. Would a prosecutor be willing to forego prosecution of the criminal under these conditions? If so, is that justice? It seems more like sympathy or mercy extended to the criminal than it does the exercise of justice. Of course, if one tries to introduce a surrogate who suffers in place of the criminal thereby resulting in the charges being dismissed, the issue becomes even more problematic. Let's imagine that the rapist has a twin brother who happens to be arrested by the police and put in jail by mistake. The innocent twin is beaten up by the inmates and is paralyzed. The guilty twin is devastated and he is not only remorseful for his act of rape but he feels terrible because it has resulted in the suffering of his innocent brother. Would the prosecutor be willing to drop charges against the guilty brother? How could the suffering inflicted upon the innocent twin pay the price for the crime committed by the guilty twin, even though the guilty twin is now suffering due to the harm inflicted on his innocent brother? It seems at best this could result in sympathy for the the twins but it could not satisfy the penalty owed by the guilty twin.
Let me offer a third scenario which may even more closely parallel Aquinas' view of the atonement. Lets say that I am the innocent twin and that from early childhood my twin brother has raped me. When I turn 18, I decide to tell my father about it. I tell my father that I love my brother and wish that he would be remorseful and repentant about his evil acts against me. My father comes up with a plan. He suggests that I report the crime to the police and that when the police come to arrest my brother, I will conveniently be at his house and will have sent my brother out on an errand. I do not lie to the police when they come but I know that they will mistake me for my brother. I am put in jail awaiting trial and the inmates beat me to the point that I am paralyzed. When my brother realizes what has happened, he comes to the police department and confesses to his crime and is extremely remorseful and repentant for his evil actions of rape. I, the police and my father are all satisfied that enough suffering has taken place and thus charges are dropped against my brother and we all forgive him. A couple of days later I am miraculously healed from my paralysis.
Now this is a very strange case but it seems to me that it pretty closely models the idea of the atonement that Aquinas had in mind. What type of justice would this be? What would we think of a father who came up with this plan? Is this model really consistent with the way the Bible portrays the Atonement? I don't think it is.
So, while I think that Aquinas' view of the atonement escapes some of the problems of the PST, I think at the end of the day, it still fails to explain how the death of an innocent person can satisfy the penalty owed by the guilty and I am not sure it accurately reflects the teaching of Scripture.