I have to confess that one question posed by TMK, and it is one worth giving time to, is new to me. I never thought of -- and frankly never heard the criticisms from either unbelievers or those who reject PSA -- the question, "Was God unjust in laying our sins on the innocent Jesus?" There is this much of TMK's "devil's advocate" question that remains unaddressed in the thread, so far as I can see:
"You can call what God did with Jesus whatever you want, but you can't call it justice because then you are just re-defining the word. Just is when a person gets what they deserve. ... WE are the ones who deserved to be punished, not Jesus. But Jesus was punished for OUR crimes. It is truly wonderful, and yes he was a substitute, but it wasn't just. That is why some people object to the idea of penal substitution and lean toward Christus Victor because that view takes justice out of the equation."
Now, y'all, that is as fair and direct a way of putting the issue as we could want. Fact is, we generally (most believers, I think) do not have in our equipment box a succinct or satisfying way of answering this objection. Now, in all fairness, an answer need not be succinct or even satisfying in order to be true, but given our admonition to be ready to give an explanation for our hope, it is something that I believe the Holy Spirit and the Word of God are already prepared to reveal to us the truth of the matter. Not that many/most would be convinced, and I am hopeful and confident of God's grace for those who still think in terms of a "ransom theory" or "Christus victor", etc.
I don't back away from anything I've said, but it still doesn't arrive at the answer to the question, and I think TMK is correct in pointing that out. I'd like to skip away from the discussion thinking we've exhausted the objection, but I don't think we have even really wrapped our arms around it yet. The good news is -- at least I hope -- that the issue does not require greater minds than ours to comprehend it since we also have the Holy Spirit to reveal all truth to us.
There is a measure of something in my mind that reasons from the Bible thusly -- There are some things to great for me to comprehend, and who am I to say to God that He owes me an explanation of why He chose to do what He did in Christ. Over the years, in my personal walk and life, I have abandoned prayers and meditations and inner questions that use the word "why". Truly, I stopped years ago asking God "why" about anything. It has been liberating, and it has helped me to trust Him and to ask other questions such as "what, Lord", "how, Lord", "when, Lord", "is this right way you want me to do this, Lord", etc. I will share a little bit of my home life on this ---- my wife is still a "why"-er. She is burdened, it seems to me, with the need to know "why" about most everything. It is her inclination to draw conclusions as to "why" this or that happens. It is also a source of frustration for her because I don't. I start with the assumption from Scripture that God is doing, and that He is working things together for the good of those who love Him and are His called for His purposes. There are many mysteries as to the reasons for things happening and for the words and actions of people; I don't waste time asking for those reasons anymore. In that same line of thinking, in my own admitted bias that may indeed be wrong but I think it is not wrong, I come to this question put by TMK. Who am I to ask Him why?
And, in this thinking, I find some warrant for it in Romans 9, and I am unsure how much it fairly bears on TMK's question but I think it might somewhat.
There, to shorten this already long post, Paul argued regarding God's sovereign choice of Israel as His people, that when Jacob and Esau were yet in the womb, "though they were not yet born and had done nothing either good or bad -- in order that God's purpose of election might continue, not because of works but because of Him who calls -- [Rebekah] was told, 'The older will serve the younger'. As it is written, 'Jacob I loved, but Esau I hated'."
He went on: "What shall we say then? Is there injustice on God's part? By no means! For He says to Moses, 'I will have mercy on whom I will have mercy, and I will have compassion on whom I have compassion'. So then it depends not on human will or exertion, but on God who has mercy." He goes on to say that God even raised Pharaoh up in a way we may humanly call arbitrary, but, as I alluded to earlier about glory, Paul points out that God said Pharaoh's ascenion and arrival at conflict with Israel was by God "that [He] might show [His] power in [Pharaoh], and that [His] name might be proclaimed in all the earth."
Then, Paul comes to where I can understand a little something about it -- "You will say to me then, 'Why does He still find fault? For who can resist His will?' But who are you, O man, to answer back to God?" He continues and concludes, essentially, that it is just because God selects His own ways, and that He is not unjust in His ways even if we cannot assent to them. He says of those who do not assent to them, "They have stumbled over the stumbling stone."
Admittedly, this passage does not address itself to the crucifixion directly. But, it is through the crucifixion that Jesus actually makes effective His covenant of promise with Israel, His elect. And, there is an element here of "who are you, O man, to answer back to God" when the Word says there is no injustice with Him.
So, we start with that conclusive premise -- God is just, and He is just in electing Israel, and He is just in the means of establishing the covenant with Israel that is of faith through the blood of Jesus, which happened on Calvary.
Now, let me ask this as an intermediate question. Would God be unjust if He did not do what He said He would do? I submit to you that the answer is 'yes'. I submit to you that the very meaning of the word 'righteousness' means God doing what God said He would do, and that our 'righteousness' is our God-enabled obedience, our doing what He says. Though there is much more to be said about righteousness, let's avoid that rabbit trail. The question is asked only for this point -- If God did not do what He said He would do, He would be unrighteous and, therefore, unjust.
God chose Israel and was just in doing so and the Bible clearly teaches us we have no standing to ask why not some other way. God also chose His Son's death on the cross as the means of achieving His covenant with Israel AND achieving what He said He would do regarding the writing of His law on the hearts of those who would believe. If God is just in the one, how is he unjust in the other?
Is justice found in the means? Or, is it found in the motivation? I ask this because God's motivation was to write His law upon our hearts, to make us inwardly empowered and able to meet the righteous requirements of the law. "I will give you a new heart and put a new spirit within you; and I will remove the heart of stone from your flesh and give you a heart of flesh. I will put My Spirit within you and cause you to walk in My statutes, and you will be careful to observe My ordinances." Ezekiel 36:26, 27. "For what the Law could not do, weak as it was through the flesh, God did: sending His own Son in the likeness of sinful flesh and as an offering for sin, He condemned sin in the flesh, so that the requirement of the Law might be fulfilled in us, who do not walk according to the flesh but according to the Spirit." Romans 8:3, 4.
It seems to me that God is just in His motivation, and sovereign in choosing the means of accomplishing His purposes.
TMK, is the question then posed by you -- and if not by you then by those who reject PSA on the grounds of injustice or from the ability to reconcile the means of God through the cross with God's justness -- really about what seems to be unjust means? Isn't the motivation itself wholly just? And if so, and if there is no other means to arrive at that end, doesn't the means that God chose also meet even the very humanly imperfect definition of justice?