"Jesus' blood 'propitiated' or satisfied God’s wrath (1:18), so that his holiness was not compromised in forgiving sinners. Some scholars have argued that the word propitiation should be translated expiation (the wiping away of sin), but the word cannot be restricted to the wiping away of sins as it also refers to the satisfaction or appeasement of God’s wrath, turning it to favor (cf. note on John 18:11). God’s righteous anger needed to be appeased before sin could be forgiven, and God in his love sent his Son (who offered himself willingly) to satisfy God’s holy anger against sin. In this way God demonstrated his righteousness, which here refers particularly to his holiness and justice. God’s justice was called into question because in his patience he had overlooked former sins. In other words, how could God as the utterly Holy One tolerate human sin without inflicting full punishment on human beings immediately? Paul’s answer is that God looked forward to the cross of Christ where the full payment for the guilt of sin would be made, where Christ would die in the place of sinners. In the OT, propitiation (or the complete satisfaction of the wrath of God) is symbolically foreshadowed in several incidents: e.g., Ex. 32:11–14; Num. 25:8, 11; Josh. 7:25–26."
Crossway Bibles (2009-04-09). ESV Study Bible (Kindle Locations 277848-277859). Good News Publishers/Crossway Books. Kindle Edition.