As an atoning sacrifice (Greek hilastērion): This Greek word appears in the Greek Old Testament. It refers to the atonement cover, the lid placed on the ark of the covenant inside the most sacred room of the tabernacle. This cover played a central role in the Day of Atonement ceremony (Leviticus 16). Over time, the word came to represent the atonement ritual itself.
Paul uses this word to describe Jesus Christ. God provided Jesus as the final sacrifice for sin. Through Jesus, God deals fully with human sin. Jesus bears God’s righteous anger against sin himself (see Romans 1:18).
the sins committed beforehand: Paul refers to faithful people in the Old Testament period whose sins were not punished as strict justice would require. The letter to the Hebrews explains that animal sacrifices could not truly remove sin (Hebrews 10:4).
So how could God forgive people before Christ? Paul explains that Jesus’s sacrifice works both backward and forward in time. Through Christ, God fully satisfied his righteous judgment against human sin for all people.