TGP Distinctives: Motivation
it refers specifically to the “mercy seat,” i.e., the covering of the ark where the blood was sprinkled in the OT ritual on the Day of Atonement (Yom Kippur). This term is used only one other time in the NT: Heb 9:5, where it is rendered “mercy seat.” There it describes the altar in the most holy place (holy of holies). Thus Paul is saying that God displayed Jesus as the “mercy seat,” the place where propitiation was accomplished.
In English translations, it’s difficult to see that “justify” (δικαιόω; dikaioō) and “righteousness” (δικαιοσύνη; dikaiosynē) share the same Greek root. In Paul’s day, this terminology would have been common in a court of law, where it was used by the judge to declare an accused person “not guilty.”
Paul uses justification language in a similar way to explain how, through faith, the work of Christ makes it possible for sinners to be found righteous by God. This explains why justification and faith are mentioned together so often. In 3:20, 28, Paul explains that being justified through faith is the opposite of being justified by works.