Are the Christian doctrines of imputation and substitution in which our sin is counted to Christ and His righteousness is credited to us, unjust? How can a just judge (God) acquit a guilty defendant (sinners) and punish an innocent party (Christ)? How is it just for the innocent to suffer for the sins of the guilty?
First, it is helpful to recognize that human analogies fall short and are thus misleading. Surely it would be unjust for a human judge to exonerate a murderer and to instead punish some innocent bystander. But the fact that it would be unjust in the human realm, does not necessitate that it is unjust in the divine. Why not? Because our sin is not ultimately against some abstract concept of justice or societal mores, but rather against God Himself. At its core, our offense is directed primarily not against a principle, but rather a person (or to be more trinitarianly precise, 3 persons: Father, Son, and Spirit). Our sin has personally offended the Judge and the Substitute. If the offended and innocent parties agree that substitution is just and good, then who are we to argue otherwise?
Second, the Scriptures do not conceive of imputation as being a problem, but instead a solution. Rather than calling into question God’s justice, the work of Christ as a propitiatory sacrifice actually evidences God’s justice.
But now the righteousness of God has been manifested apart from the law, although the Law and the Prophets bear witness to it— 22 the righteousness of God through faith in Jesus Christ for all who believe. For there is no distinction: 23 for all have sinned and fall short of the glory of God, 24 and are justified by his grace as a gift, through the redemption that is in Christ Jesus, 25 whom God put forward as a propitiation by his blood, to be received by faith. This was to show God’s righteousness, because in his divine forbearance he had passed over former sins. 26 It was to show his righteousness at the present time, so that he might be just and the justifier of the one who has faith in Jesus.
According to Paul, God’s righteousness (δικαιοσνη: the same Greek root as “just” and “justification”) is manifested, displayed, evidenced, shown and demonstrated in the act of the atonement whereby Jesus Christ bore our sin and imputed to us His righteousness through the instrument of faith uniting us to Him. God’s justice is not obscured by substitution; it is exposed.
How so? Because God “had passed over former sins.” Paul will later write in Romans 4 quoting Psalm 32, “blessed is the man against whom the Lord will not count his sin.” God simply passes over sins and does not count them against His children. Remember Nathan’s confrontation of David after the events of his adultery with Bathsheba and murder of Uriah? David confesses that he has sinned and is guilty and how does Nathan respond? “The LORD also has put away your sin.” How can the LORD simply “put away” David’s sin? Isn’t this unjust? This is exactly the type of question that Romans 3 answers. It is exactly this question that the gospel answers.
In punishing His Son, God demonstrates His righteousness and justice. He displays both His righteous wrath against sin and also His righteous acceptance of unrighteous men and women. He evidences how He can both be just and yet also justify the unjust. Substitutionary atonement does not contradict God’s righteousness; it confirms it.
For our sake he made him to be sin who knew no sin, so that in him we might become the righteousness of God.
2 Corinthians 5:21