Jeremiah was commissioned by God to preach everything God told him. He preached his entire life. He converted no one.
God knew Jeremiah would not be effective, so why have him preach? So the Jews would have no excuse. They could not say, "I was never told".
But Jeremiah wasn't the first.
Moses and Aaron were commanded to communicate to Pharaoh everything God had said. But God advised them He would inhibit Pharaoh from complying.
So "I can multiply MY signs and MY wonders. I AM THE LORD." Exodus 7:2-5
The Egyptians would be without excuse.
What has this have to do with you and me?
We are the "Jeremiah's" of this generation. Like Moses and Aaron, we tell unlikely respondents about God's great works. And like Moses, we tell them again, and again, and again. They NEED to know who the Lord is.
Who will you speak to today?