God is made angry when a person sins. The word “propitiation” is used to talk about doing something so that God stops being angry.
In the Old Testament, this was done by offering a sacrifice to God. God needed to punish people for sinning because God is holy and just. When a person sinned, he was able to offer a sacrifice to God. God accepted this sacrifice and forgave the person for sinning. In this way, God remained just and he was no longer angry.
In the New Testament, Jesus is the propitiation for people’s sins (see: Romans 3:25; Hebrews 2:17; 1 John 2:2; 4:10). Jesus was punished for other people’s sins when he died on the cross. This allowed God to stop being angry with people for their sins. Jesus’ sacrifice made it possible for God to show kindness to people and to allow those who believe in Jesus to live with him forever.