The justice of YHWH is as unlike the retributive justice of our legal system as Jesus of Nazareth was unlike the Pharisees. The ancient Hebrew understanding of justice is the antithesis of all conventional and orthodox concepts of retributive or payback justice. The Christian church has displaced the justice of YHWH with the Latin (Roman) concept of retributive justice and used that as a foundation for atonement (payback) theology.
As a result, Jesus' self-sacrifice on the cross is seen merely as a response to the sin problem that required satisfaction or atonement for the transgression of God's law and offended holiness/honor. God can only be merciful to sinners as long as someone (Jesus) has paid the penalty and atoned for their sin. Nothing could be further from the truth.
If the biblical concept of justice was retributive and punitive (a person gets what he deserves) this would be true. But there is no tension between justice and mercy in the biblical concept of God's justice. I would like to quote from the excellent article by Robert Brinsmead, The Scandal of God's Justice, "God's justice is love in action. Therefore it is not justice in tension with mercy, but justice expressed in mercy. It is not justice which is punitive, but justice which brings salvation to those who sit on the dunghill of human misery. It is not justice which augers doom and gloom, but justice which calls for celebration with singing, feasting and dancing."
Once again it is instructive to go back to the original language of the Bible to find the true meaning of God's justice. The Hebrew word sadaq and the Greek word dikaiosune, which translate to justice in English. The biblical concept of justice should not be confused with the conventional idea of justice. The conventional concept of justice is expressed in our legal system: a person is punished for doing wrong. Mercy is thought to be the opposite of justice and in tension with it. The word sadaq (justice) has to do with loyalty to a relationship. To quote Brinsmead once again, "When a party fulfills the demands of a relationship, that party conforms to what ought to be. Then a state of sadaq (justice) exists. Thus, sadaq 'concerns the right order of things' - i.e. the correct ordering of the world according to the divine intention." What is the divine intention? What is the will of God? I Timothy 2:4, "...our Savior, God, Who wills that all mankind be saved and come into the realization of the truth."
God's justice is salvific, its purpose is to restore the relationship between us and God. The right order of things is for all of us to become as truly human as the Human One: Jesus Christ - the image of God. God is just, makes things right, by being unconditionally faithful to His covenant promise to us. As Brinsmead observers, " God is faithful to his covenant promise. God's covenant is not a conditional contract bilaterally concluded by two parties. Its is a unilateral commitment or promise on God's part to act toward his chosen covenant partner with overwhelming kindness and generosity. From the beginning God has had a purpose of grace toward mankind (2 Tim. 1:9). He has had a commitment to fulfill this gracious purpose at any cost to Himself. Thus, when God exercises His saving mercy toward sinful people, He is simply fulfilling His covenant promise. Justice is God faithfully carrying out just what divine love had pledged to do." God will fulfill His covenant with us and the entire creation no matter what it costs Him. The self-emptying of God on the cross of Jesus Christ is the justice of God revealed. On the cross He suffered true death and damnation so that everything would be set right, so that justice would be fulfilled. "Jesus so fully identified with the suppressed, the depressed and the oppressed that He bore their curse and experienced their rejection. He also knew what it meant to be deprived of justice ... In all this, Jesus revealed what God always was and ever shall be. He is on the side of all who are deprived and oppressed - so fully on their side that He became such Himself." (Brinsmead)
The death of Jesus on the cross is not God's emergency response to salvage a small portion of humanity and the creation from the trash heap of sin and death. It is God's proactive act of unconditional kindness and love, who never abandons us but rather is Emmanuel: God with us. In Jesus' death on the cross we see the full self-revelation (glory) of God. God identifies with the godless and godforsaken and becomes what God is not: sin (selfishness) and death. And by doing so God is true to Himself because no greater love is there then to lay down your life for those who hate and despise you, or are indifferent towards you. This is true justice: sadaq, YHWH makes all things right by unilaterally remaining faithful to His covenantal promise to us and all creation. He does this by sparing no cost to himself; from the beginning He was self-emptying love: The lamb slain from the foundation of the cosmos.