Given that sin is not a debt - it's a crime against God - why do some say God only had to forgive? What about justice? Is God not just?

Muslim here, although I don't think I can put a full answer but here's what I know:

Matthew 20:28 Euen as the Sonne of man came not to be ministred vnto, but minister, and to giue his life a ransome for many.

So while this can be linked to the crucifixion, we still have a problem related to Ezekiel 18:

Ezekiel 18:20 The soule that sinneth, it shal die: the sonne shall not beare the iniquitie of the father, neither shal the father beare the iniquitie of the sonne; the righteousnesse of the righteous shall bee vpon him, and the wickednesse of the wicked shalbe vpon him.

So it's clear that God doesn't burden a man with the sins of another, so where's the problem?

There's difference between Paul's teachings and what we read in John 5:

28 Marueile not at this: for the houre is comming, in the which all that are in the graues shall heare his voice,

29 And shall come foorth, they that haue done good, vnto the resurrection of life, and they that haue done euill, vnto the resurrection of damnation.

Again, this doesn't confirm the salvation based-on-faith according to Galatians 2:16, since it's clear that everyone will be recompensed for their faith and also their deeds.

So can someone ransom his life then come back without diminishing the sacrifice?

Evidently [as a non-Christian] God is a Perfect Judge, but it looks like the different interpretations of the doctrine is the reason behind the denominations existence in the first place.

Can we really expect no punishment in any form for crimes against God?

Do you think that God will leave someone like Hitler without a verdict?

Matthew 18:7 ¶ Woe vnto the world because of offences: for it must needs be that offences come: but wo to that man by whom the offence commeth.

The denominations is not the answer you are seeking, God doesn't change no matter what we believe about Him, don't forget that:

Isaiah 46:9 Remember the former things of old, for I am God, and there is none else, I am God, and there is none like me,

From C. S. Lewis' Mere Christianity, Book II What Christians Believe

Now before I became a Christian I was under the impression that the first thing Christians had to believe was one particular theory as to what the point of this dying was. According to that theory God wanted to punish men for having deserted and joined the Great Rebel, but Christ volunteered to be punished instead, and so God let us off. Now I admit that even this theory does not seem to me quite so immoral and so silly as it used to; but that is not the point I want to make. What I came to see later on was that neither this theory nor any other is Christianity.

The central Christian belief is that Christ's death has somehow put us right with God and given us a fresh start. Theories as to how it did this are another matter. A good many different theories have been held as to how it works; what all Christians are agreed on is that it does work. ... Theories about Christ's death are not Christianity: they are explanations about how it works. Christians would not all agree as to how important these theories are.

My own church—the Church of England—does not lay down any one of them as the right one. The Church of Rome goes a bit further. But I think they will all agree that the thing itself is infinitely more important than any explanations that theologians have produced. I think they would probably admit that no explanation will ever be quite adequate to the reality. But as I said in the preface to this book, I am only a layman, and at this point we are getting into deep water. I can only tell you, for what it is worth, how I, personally, look at the matter.

On my view the theories are not themselves the thing you are asked to accept. Many of you no doubt have read Jeans or Eddington. What they do when they want to explain the atom, or something of that sort, is to give you a description out of which you can make a mental picture. But then they warn you that this picture is not what the scientists actually believe. What the scientists believe is a mathematical formula. The pictures are there only to help you to understand the formula. They are not really true in the way the formula is; they do not give you the real thing but only something more or less like it. They are only meant to help, and if they do not help you can drop them. The thing itself cannot be pictured, it can only be expressed mathematically.

We are in the same boat here. We believe that the death of Christ is just that point in history at which something absolutely unimaginable from outside shows through into our own world. And if we cannot picture even the atoms of which our own world is built, of course we are not going to be able to picture this. Indeed, if we found that we could fully understand it, that very fact would show it was not what it professes to be—the inconceivable, the uncreated, the thing from beyond nature, striking down into nature like lightning. You may ask what good will it be to us if we do not understand it. But that is easily answered. A man can eat his dinner without understanding exactly how food nourishes him. A man can accept what Christ has done without knowing how it works: indeed, he certainly would not know how it works until he has accepted it.

We are told that Christ was killed for us, that His death has washed out our sins, and that by dying He disabled death itself. That is the formula. That is Christianity. That is what has to be believed. Any theories we build up as to how Christ's death did all this are, in my view, quite secondary: mere plans or diagrams to be left alone if they do not help us, and, even if they do help us, not to be confused with the thing itself. All the same, some of these theories are worth looking at.

The one most people have heard is the one I mentioned before —the one about our being let off because Christ had volunteered to bear a punishment instead of us. Now on the face of it that is a very silly theory. If God was prepared to let us off, why on earth did He not do so? And what possible point could there be in punishing an innocent person instead? None at all that I can see, if you are thinking of punishment in the police-court sense. On the other hand, if you think of a debt, there is plenty of point in a person who has some assets paying it on behalf of someone who has not. Or if you take "paying the penalty," not in the sense of being punished, but in the more general sense of "standing the racket" or "footing the bill," then, of course, it is a matter of common experience that, when one person has got himself into a hole, the trouble of getting him out usually falls on a kind friend.

Now what was the sort of "hole" man had got himself into? He had tried to set up on his own, to behave as if he belonged to himself. In other words, fallen man is not simply an imperfect creature who needs improvement: he is a rebel who must lay down his arms. Laying down your arms, surrendering, saying you are sorry, realising that you have been on the wrong track and getting ready to start life over again from the ground floor—that is the only way out of a "hole." This process of surrender—this movement full speed astern—is what Christians call repentance. Now repentance is no fun at all.

It is something much harder than merely eating humble pie. It means unlearning all the self-conceit and self-will that we have been training ourselves into for thousands of years. It means killing part of yourself, undergoing a kind of death. In fact, it needs a good man to repent. And here comes the catch. Only a bad person needs to repent: only a good person can repent perfectly. The worse you are the more you need it and the less you can do it. The only person who could do it perfectly would be a perfect person—and he would not need it.

Remember, this repentance, this willing submission to humiliation and a kind of death, is not something God demands of you before He will take you back and which He could let you off if He chose: it is simply a description of what going back to Him is like. If you ask God to take you back without it, you are really asking Him to let you go back without going back. It cannot happen. Very well, then, we must go through with it. But the same badness which makes us need it, makes us unable to do it. Can we do it if God helps us? Yes, but what do we mean when we talk of God helping us? We mean God putting into us a bit of Himself, so to speak. He lends us a little of His reasoning powers and that is how we think: He puts a little of His love into us and that is how we love one another.

When you teach a child writing, you hold its hand while it forms the letters: that is, it forms the letters because you are forming them. We love and reason because God loves and reasons and holds our hand while we do it. Now if we had not fallen, that would be all plain sailing. But unfortunately we now need God's help in order to do something which God, in His own nature, never does at all—to surrender, to suffer, to submit, to die. Nothing in God's nature corresponds to this process at all. So that the one road for which we now need God's leadership most of all is a road God, in His own nature, has never walked. God can share only what He has: this thing, in His own nature, He has not.

But supposing God became a man—suppose our human nature which can suffer and die was amalgamated with God's nature in one person—then that person could help us. He could surrender His will, and suffer and die, because He was man; and He could do it perfectly because He was God. You and I can go through this process only if God does it in us; but God can do it only if He becomes man. Our attempts at this dying will succeed only if we men share in God's dying, just as our thinking can succeed only because it is a drop out of the ocean of His intelligence: but we cannot share God's dying unless God dies; and He cannot die except by being a man. That is the sense in which He pays our debt, and suffers for us what He Himself need not suffer at all...

Such is my own way of looking at what Christians call the Atonement. But remember this is only one more picture. Do not mistake it for the thing itself: and if it does not help you, drop it.

Another possibility is that God is merciful enough that He is willing to just waive the damages/punishment but that any person who is truly willing to turn back to God after sinning is someone who would be uncomfortable with the damages they have caused and thus part of God's mercy is providing us with a mechanism so we can feel that justice has occurred as well. Or that without God's judgement, we would not know we needed mercy. I'm sure there are plenty of other ways of looking at this that which do not require pitting mercy and justice against each other.

Would it be right for any Judge to wave a hand and say, "Yes you are guilty of all those crimes but I'm just going to provide you with a free pardon. I expect you to be so grateful, you will then be a reformed character who will start to do good instead of doing evil" ?

Yes. But only if you add the condition that the victims of the crimes be suitably compensated.

For the Israelites, justice generally meant compensating the victim, not being punished in the modern sense of the word.

Israel had no jails or prisons. When someone caused loss to some other person, they were expected to repay the value of that loss. If they couldn't afford it, they sold themselves as bondsmen in order to raise the money. They effectively became slaves for a month or a year or whatever it took to repay the debt.

People quickly and effectively learned to follow the rules. Not because they were afraid of jail or torture, but because they learned the true cost of their actions.

(Our modern justice systems seem to have almost completely lost this concept of reform and rehabilitation. Punishment and suffering are what the public seems to want and get.)

But that system was for the physical nation of Israel, where breaking the law, sinning, meant causing harm or loss to other people, where compensating the victim literally cancelled the original damage.

For Christians though, sins are crimes against God. It is not possible for a mortal human to compensate God for what one has done.

The cost of sin is death, and there is nothing one can do to pay that penalty other than by losing one's life.

To think that one can "do penance", or personally suffer pain, or do anything at all to compensate God is blasphemous. Such ideas make a joke of Jesus's sacrifice. God doesn't want people to suffer, he wants them to understand, to improve their characters, to repent of their behaviour.

God's grace and forgiveness is a gift, but it must first be deserved and wanted. No one can earn salvation, but one can choose to remain unworthy to receive it.

God's judgement isn't "fair". If it were truly fair, everyone would die and no one would be saved.

When sinning against God, there is no such thing as a small sin or a large sin. One either sins or one doesn't. One either repents of their sin or they don't. It's that simple.

What about justice? Is God not just?

2 Peter 2:9 talks about punishment at the day of judgement:

then the Lord knows how to deliver the godly out of temptations and to reserve the unjust under punishment for the day of judgment,

In secular trials, first there is a judgement as to guilt, and then there is a judgement as to degree of punishment. But that is not how God's judgement works.

God won't torture the "bad, but not that bad" sinners for a few thousand years until they've suffered enough to "purge" their sins. One either repents of sin and asks God for forgiveness or one doesn't. There is no middle ground. One either receives salvation or one doesn't.

Will God torture the unrepentant for all eternity? Will the unrepentant receive an everlasting life of torment?

Rather than repeating the verse mindlessly, read what John 3:16 actually says about this:

For God so loved the world that He gave His only begotten Son, that whoever believes in Him should not perish but have everlasting life.

The saved will receive "everlasting life", and the rest will "perish". There's no third alternative, no compromise; it's either everything or nothing.

God is a god of love, who "desires all men to be saved and to come to the knowledge of the truth". People that think of God as sadistic or vindictive are confusing him with Satan and worshiping the wrong god. Those that reject his salvation will not have to suffer torture for all eternity; they will simply be destroyed.

The word in 2 Peter 2:9 translated as "punishment" is "κολαζομένους" G2849 - kolazō - Strong's Greek Lexicon. Its primary meaning is "properly, to lop, prune, as trees, wings.".

Those that reject God's salvation will be pruned away like the useless parts of a productive plant. As Matthew 3:12 says in another agricultural metaphor, God will "gather His wheat into the barn; but He will burn up the chaff with unquenchable fire".

No one deserves salvation more than anyone else. In fact, no one "deserves" salvation at all; it can't be earned but is freely given to those that want it.

Permanent death and destruction is God's punishment for the unsaved. Everlasting life and blessings are God's gift (not reward) for those that truly want it.

That is the justice of a loving God.