Why did Hashem say that He will make Yirmeyah a copper wall?
Radak: I will not be to you like Achzav (one who you cannot rely on), rather, like I told you initially ("a fortified city, iron pillar and copper wall" - 1:18).
Why did Hashem say that they will fight you, and they cannot overcome you?
Radak: I did not tell you that their war against you will be temporary. You did not understand My words well. I will fulfill My promise. They cannot overcome you to kill you, but they will fight you via words, blows and disgrace, as long as You rebuke them. You will endure all this suffering to return them to Me, for if you do not return all of them, it is impossible that you will not return some of them. We find that when they wanted to kill him, the officers said "he is not Chayav Misah. He spoke to us in the name of Hashem Elokeinu" (26:15). If you will toil for this and not be concerned for their verbal abuse, you will receive reward from Me.
Malbim: I will save you via Hashgachah.
