Hilastērion appears only twice in the NT (Romans 3:25; Hebrews 9:5) but carries enormous theological weight. In Hebrews 9:5 it clearly refers to the mercy seat (kappōret, H3727) of the Ark of the Covenant — the golden cover over the ark in the Holy of Holies where the high priest sprinkled blood on Yom Kippur. In Romans 3:25, Paul says God publicly set forth Christ as a hilastērion — either 'a mercy seat' (God's throne of grace where atonement is made) or 'a propitiation' (the means by which God's wrath against sin is satisfied). Both senses are theologically valid and complementary.
Romans 3:25 is one of the most theologically dense sentences in the NT: 'God presented Christ as a hilastērion, through the shedding of his blood — to be received by faith. He did this to demonstrate his righteousness, because in his forbearance he had left the sins committed beforehand unpunished.' Paul is saying that Christ on the cross is the ultimate Yom Kippur — the final, permanent mercy seat where God's justice (requiring punishment for sin) and his mercy (forgiving the penitent) meet in the blood of his own Son. The cross is where steadfast love and faithfulness meet; where righteousness and peace kiss each other (Psalm 85:10). Christ is not merely the sacrificial animal but the mercy seat itself.