ISB ~ Romans 3 v25

an expiation:A sacrifice that wipes away sin.

• The expression is used multiple times in the Greek OT for the mercy seat, or golden lid that covered the Ark of the Covenant (Ex 25:17; Heb 9:5). The high priest of Israel sprinkled blood on the mercy seat once a year on the Day of Atonement to expiate the sins of the people and restore them to fellowship with Yahweh (Lev 16:1-34). For Paul, the mercy seat typifies Christ as the living seat of God’s presence and the place where atonement is made with sacrificial blood (CCC 433).

• Christ, who became an expiation by blood, teaches us to follow his example by the mortification of our members (St. Gregory of Nyssa, On Perfection).