“I will cleanse them from all their iniquity by which they have sinned against Me, and I will pardon all their iniquities by which they have sinned and by which they have transgressed against Me.”
While imprisoned for prophesying that Jerusalem would fall to the siege by Nebuchadnezzar (Jeremiah 32:1-5; 33:1), Jeremiah also prophesied of the good news beyond all the bad. In the future, God would bring both Judah and the other tribes of Israel back from captivity (Jeremiah 33:7). (Only Judah returned historically. This is a prophecy describing one of the events at the end times when Jesus Christ will return.)
Then God shared even better news. He would make it possible to not only remove the penalty of sin, but to also cleanse us from the blotch and horrible stain that sin leaves on us.
This cleansing and forgiveness, of course, was made possible by the sacrifice of Jesus Christ, and it is only available to those who repent (John 1:29; 1 John 1:9; Acts 2:38). David demonstrated sincere repentance when he asked God to “wash me thoroughly from my iniquity, and cleanse me from my sin” (Psalm 51:2).
For more about repentance, see “What Is Repentance?”