In the little Epistle to Philemon, we have Paul's side of his relationship with a wealthy man. Part of that relationship involves a slave named Onesimus. Onesimus (Greek for "useful") had run away, during which time Paul had met him. Paul convinced him to become a Christian, and to return to his place in the household of Philemon. The epistle is then Paul's plea to Philemon to receive Onesimus back, not just as a member of his household, but now also as a brother in Christ.
Paul makes a plea on behalf of Onesimus (Phmn 1:18-19): "If he has wronged you at all, or owes you anything, charge that to my account. I, Paul, write this with my own hand: I will repay it."
I think this is a very significant statement. How so? Paul is expressing the biblical concept of surety. He is obliging himself to cover any shortcomings on the part of Onesimus.
Paul provides here a pithy, visible example of the role Jesus plays for the elect. Just as Paul pledged to make up any shortfall in Onesimus, Jesus committed to the Father to cover the debts of His people.
This is justification: our debt is cancelled because Jesus has paid it all. We see how Rome's concept of justification, by an infusion of righteousness, falls short of the biblical model. Paul's commitment was to pay for Onesimus's debts, not to give money to Onesimus to pay for himself. In the same way, the Christian is justified before God not because Jesus has transferred some of His righteousness to him, but rather because Jesus has assumed the debt of sin on Himself. The justified sinner does not stand before God as now good enough, but rather as a criminal now declared "not guilty"!