John Calvin Commentary Jeremiah 1:19

"And they shall fight against thee; but they shall not prevail against thee: for I am with thee, saith Jehovah, to deliver thee." — Jeremiah 1:19 (ASV)

In this verse, God briefly reminds His servant that although he would be supplied with invincible power, he would still face great trials, so that his office would not be, according to a common saying, a mere game. God then shows for what purpose he would be made like a fortified city, an iron pillar, and a bronze wall: it was so that he might fight manfully, not to keep away all dangers, all conflicts, and everything hard and painful to endure. In short, we see that the promise was given for this purpose—that Jeremiah, relying on God’s aid, might not hesitate to set himself against all the Jews, and that whatever their fury might be, he might still be courageous.

Now, a profitable doctrine may be gathered from this: whenever God promises His servants victory over their enemies, they ought not to use this as an occasion to foster their lethargy or idleness. On the contrary, they should gather courage, so that they may proceed boldly and tirelessly in the course of their calling. In short, God promises to be their deliverer, but at the same time exhorts them to resist all the assaults of their enemies.

Hence He says, They shall fight with thee, but they shall not prevail, for I am with thee to deliver thee. From these words, we see that Jeremiah was fully armed so that he might not fear when seeing dangers surrounding him. For God does not declare here that He would be like a wall to him to prevent him from being assaulted; rather, He says that He would deliver him. It is as though He had said, “Prepare yourself to suffer, for if I were not your deliverer, it would be all over for you, and you might perish a hundred times. But there is no reason for you to fear any dangers amid a thousand deaths, since I am present with you as your deliverer.”