From a human perspective, there are two ways to retire a debt that a person cannot afford to pay. One way is for the debtor to be declared bankrupt by a court because the debtor’s liabilities exceed his assets. The debt owed by the debtor exceeds his ability to pay. In this scenario, the creditor must write off what is owed. Although our laws make this type of debt-relief possible, in the strictest sense of the word, justice has not been done because what is owed has not been paid. Another way for the debt to be dealt with is for a third party to pay off the debt for the debtor. The debt is not written off, it is paid off. The debtor is released from the debt because of the actions of a third party. The third party has assumed the debt of the debtor and performed on the debt as if it were his own. Justice is done because the debt is paid.
The second scenario is a perfect picture of what Jesus did on the cross. He who was without sin (had no debt of His own) became sin on our behalf (took on our debt) and died in our place (paid the debt) as if our debt were His own. While we were yet sinners (debtors with no ability to pay), Christ died for us (paid our debt). Justice was done because our debt was paid in full