Hardened by sin

 Jeremiah 13:23–24 (NRSV)

23Can Ethiopians change their skin or leopards their spots? Then also you can do good who are accustomed to do evil.
24I will scatter you like chaff driven by the wind from the desert.

The nation of Israel was a stubborn people who refused to change their ways. They were set on worshiping other gods, and nothing would cause them to repent and return to Jehovah. They were the nation that God would draw near to them and they in turn would cling to him out of love for him and what he had done for them. But they became attached to the false gods who could do nothing for them. The insanity of sin is seen in the vile behavior of people intent on doing evil. They become so corrupt that doing good is foreign to them. It will take extreme measures to bring about changes in the conduct of the people. For Israel this would mean captivity and destruction of the cities they inhabited. The land God had given to them was polluted by their sinfulness and God had to remove them from it. The nation would be scattered abroad and after 70 years of Babylonian captivity only a remnant would be permitted to return to Jerusalem. In spite of all of the warnings and demonstrations that Jeremiah would use to communicate the coming judgment, it had no effect. Sin had hardened their hearts, and change was impossible until the judgement fell and the people realized the gravity of their rebellion and were willing to repent.