God put forward Christ Jesus as a propitiation by his blood, to be received by faith. This was to show God's righteousness, because in his divine forbearance he had passed over former sins.
Romans 3:25

Propitiation means a cleansing or wiping away. Septuagint uses it to refer to the kiporet of the Ark of the Covenant. But his blood doesn’t just cover over sins. It removes them. See Rev 7:14. This forgiveness shows God’s righteousness, because only one who is perfectly sinless could be qualified to completely remove all sin, unlike animal sacrifices which only temporarily covered them over.