Jeremiah 4 was a warning to Israel of the coming invasion by the Babylonians. Yet even in this late stage, God offers Israel hope. If they were to turn again to Him, then He would not send the Babylonians, instead sending a blessing. Israel were serving idols, their hearts were not dedicated to God. They still had priests and some semblance of religion, but it was not serving God.
Jesus also talks about the heart in Matthew 15. The Pharisees and scribes had “made void the word of God” with their traditions, and Jesus applies the words of Isaiah to them, “this people honors me with their lips, but their heart is far from me”. Jesus warns the people that “what comes out of the mouth proceeds from the heart, and this defiles a person”.
The heart of Israel was unfaithful, from the times of Jeremiah and Isaiah through to the time of Jesus. This is a warning to us as well, to be aware of what is in our heart and ensure it is dedicated to God rather than ourselves.