A crude analogy is a bookie who has a debt owed to him doesn’t particularly care who pays the debt, as long as the debt is paid and “justice” is done. God is supremely holy and just and will not allow Himself to commune with someone who is sinful UNLESS His wrath toward the sin has been propitiated (appeased). So the next question is why didn’t God use someone besides His Son? For example, in the Old Testament, why wasn’t animal sacrifice sufficient? Because an animal is merely a thing, tainted by the fall of Creation, and not on the same level as a man. So why not sacrifice a man rather than a God-in-flesh man? Because all men are sinful and unable to be pure, as all sacrifices had to be. So God chose to offer Himself as the pure, spotless lamb. As John Stott says in The Cross of Christ, “It is God himself who in holy wrath needs to be propitiated, God himself who in holy love undertook to do the propitiating and God himself who in the person of his Son died for the propitiation of our sins. Thus God took his own loving initiative to appease his own righteous anger by bearing it his own self in his own Son when he took our place and died for us. So it was justice AND grace together, not either/or.

Romans 3:22-26 “This righteousness from God comes through faith in Jesus Christ to all who believe. There is no difference, for all have sinned and fall short of the glory of God, and are justified freely by his grace through the redemption that came by Christ Jesus. God presented him as a sacrifice of atonement, through faith in his blood. He did this to demonstrate his justice, because in his forbearance he had left the sins committed beforehand unpunished— he did it to demonstrate his justice at the present time, so as to be just and the one who justifies those who have faith in Jesus.”

I understand from the perspective of grace, but how can it be termed just?”

I don’t know that I can adequately answer your question, but I’ll take a stab at it. If we go back to Genesis 3 we see a picture of the coming redemption.

10And he said, I heard thy voice in the garden, and I was afraid, because I was naked; and I hid myself.

11And he said, Who told thee that thou wast naked? Hast thou eaten of the tree, whereof I commanded thee that thou shouldest not eat?

12And the man said, The woman whom thou gavest to be with me, she gave me of the tree, and I did eat.

13And the LORD God said unto the woman, What is this that thou hast done? And the woman said, The serpent beguiled me, and I did eat.

14And the LORD God said unto the serpent, Because thou hast done this, thou art cursed above all cattle, and above every beast of the field; upon thy belly shalt thou go, and dust shalt thou eat all the days of thy life:

15And I will put enmity between thee and the woman, and between thy seed and her seed; it shall bruise thy head, and thou shalt bruise his heel.

I think verse 14 and 15 are very telling because they speak of the just punishment of Satan. We don’t tend to emphasize that very much. We talk about Adam and Eve’s punishment quite a bit, but God meted out punishment for every party involved. To get a clearer picture of how God would justly punish Satan, let’s examine what verse 15 says about his punishment. It essentially says that Jesus - the seed of the woman - would crush Satan’s newfound authority (head symbolizes authority) on earth. Consider these other parallel passages:

“But to which of the angels said he at any time, Sit on my right hand, until I make thine enemies thy footstool?” (Hebrews 1:13)

“And the God of peace shall bruise Satan under your feet shortly. The grace of our Lord Jesus Christ be with you. Amen” (Romans 16:20).

“For he must reign, till he hath put all enemies under his feet” (I Corinthians 15:25).

“Thou hast put all things in subjection under his feet. For in that he put all in subjection under him, he left nothing that is not put under him. But now we see not yet all things put under him” (Hebrews 2:8).

Jesus is the just who died for the unjust (I Peter 3:18). But I think we have largely neglected the fact that his justice was not carried out apart from the judgment of Satan. Could God, in his justice, allow Satan to steal his creation and go unpunished? No way. God had already formed a plan of redemption to buy back his wayward creation.

Furthermore, we know that Jesus is God in the flesh, and he is the express image of the Father’s person. But we must also remember that Jesus is a human; this makes him unique among all creatures, in that he is 100% God and 100% man. In his ministry as high priest, he is the mediator of the New Covenant and he can be touched with the feeling of our infirmities both because of his humanity and his compassion. And we know this quality of him, Agape, is the very nature of God. So if we are asking the question of how it is just that the just died for the unjust, we must recall the fact that God is love; his justice cannot be viewed apart from love and therefore mercy.

I wouldn’t say we are abandoning the trinity. Jesus is still God, but he is also man. He carried out his earthly ministry as a man anointed with the Holy Ghost, yet he was still the eternal Word made flesh. He had to live as a man to be “tempted in all points like as we are, yet without sin” (Hebrews 4:15). Check out this passage from Philippians 2.

5Your attitude should be the same as that of Christ Jesus:
6Who, being in very nature[a] God,
did not consider equality with God something to be grasped,
7but made himself nothing,
taking the very nature[b] of a servant,
being made in human likeness.
8And being found in appearance as a man,
he humbled himself
and became obedient to death—
even death on a cross!