One could ask: “How can it be considered Justice to allow one man to pay the penalty for another man”. The answer to how God can be both Just and Justifier can be found in no one but the person of Jesus Christ.
To start with, even in our human society there is allowances for legal representation of one person on behalf of another. In the concept of an ambassador, one person is placed in a position of authority to represent another. An ambassador has the right to speak and act on behalf of the one He/She represents.
Decisions an ambassador make are legally binding on those they represent. For example, my wife and I have signed a Power of Attorney for our adoptions, those that hold a power of attorney for us have the legal authority to make decisions on our behalf, and we are legally bound to those decisions and can be held responsible for those decisions.
Adam, as the first man, came under a covenant with God to represent all of mankind. And as our representative, Adam broke that covenant, and we took on responsibility for his actions. While this alone is enough to make us guilty, we also take on direct guilt, in choosing to stand with Adam in his disobedience through our own disobedience.
Then comes Jesus in the picture, as the second representative of mankind (the second Adam); his atonement was the result of Him taking on the legal representation for us; Jesus making a decision to honor an agreement between He and God the Father – thus allowing us through His representation to take advantage of this redemption by associating and coming into agreement with his decision to honor His covenant with God.
It is through this representative model that our penal system (and moral law) allows for the redemptive nature of Jesus. Adam as our representatives made us guilty, Jesus as our representative makes us redeemed.
However, this isn’t the end of the story. A second portion of this whole discussion comes into place when you look at what the purpose of a penalty is for.
In our country of the United States, legal penalties come about through breaking the law; and paying those penalties are to bring about restitution to those we have transgressed against. In our case, we transgress against our neighbor, which is transgressing against the local authorities, to state authorities, to governmental authorities, which in our country comes up to the President of the United States. In saying this, when we break a law in our country, we are transgressing the rule of authority of the President of the United states.
No one acting on behalf of the President of the United States has the right to forgive that transgression, however, the President of the United States (as the one who was transgressed against) does have a right to “wipe the slate clean” (we see this quite frequently with the concept of a Presidential Pardon). And yet, even in this case, it can be said that it is ‘unjust’ for the President to wipe away a transgression without proper restitution being paid, and yet, even in this injustice, our penal system allows for this.
So if we take this concept even one step further, any law broken, or any over-arching moral or universal law that is broken, transgresses the ultimate law giver: The Creator, and He and He alone has the right to pardon transgressions against Himself. The right… but not the requirement, God would be just in condemning all those who have transgressed his law, and yet, His love allows Him to pardon those who have transgressed against him. However, His justice requires that those transgressions be paid for with restitution, and this is where love comes in.
While God has the authority to wipe our transgressions away, His own nature binding Him to justice would not allow Him to let transgressions go unpunished (unlike our nature as humans that will allow this in the case of a Presidential Pardon as referenced above). Because all transgressions are ultimately against Him, He is the only one that can ultimately pardon those transgressions, and yet, because of the nature of our transgressions, we have nothing to offer in restitution to pay for the gravity of our transgressions.
Therefore, God showed both perfect love and perfect justice in providing of Himself for the restitution and redemption for His creation. And still those who do not receive redemption by bringing themselves into a state of agreement in the representation of the second Adam will still receive perfect love and perfect justice.