John Calvin Commentary Jeremiah 51:58

"Thus saith Jehovah of hosts: The broad walls of Babylon shall be utterly overthrown, and her high gates shall be burned with fire; and the peoples shall labor for vanity, and the nations for the fire; and they shall be weary." — Jeremiah 51:58 (ASV)

The Prophet again introduces God as the speaker, so that what he said might gain more attention from the Jews. For this reason, he added a eulogy to the last verse, stating that the king spoke, whose name is Jehovah of hosts. We have explained elsewhere the purpose of such expressions: namely, that people may rise above everything seen in the world when God’s power is mentioned, so that they do not try to confine it within their own limited understanding. Therefore, the Prophet now repeats God's name again, so that the Jews might receive what he announced with fitting reverence.

And what he says is, The wall of Babylon, however wide it may be, shall yet be surely demolished. We have mentioned that the walls were fifty feet wide—and these were indeed long feet—although Herodotus, as I have said, refers to cubits and not feet. Indeed, the width was so great that four horses abreast could pass each other, as there was enough space for them.

Therefore, it appears that their thickness was so great that the Babylonians confidently disregarded everything the Prophet had predicted. No war engines could have ever battered down such thick walls, especially since they were made of bricks and cemented with bitumen. Since, then, the material, in addition to the thickness, was so firm and strong, this prophecy seemed incredible. Indeed, it did not reach the Babylonians, but the Jews themselves considered everything they had heard from the Prophet's mouth to be a fable. Yet God did not refer to the wall's width in vain. He did so in order that the faithful might be assured that the walls of Babylon could not possibly resist Him, however firm they might be in their materials and thickness. The wall, he says, shall surely be demolished.

He afterwards mentions the gates, which Herodotus says were made of brass when Darius removed them. He indeed means the doors, but the Prophet includes the framework as well as the bronze doors. He then says, they shall be consumed with fire. The Babylonians might have laughed at this threat from Jeremiah, because brass could not have been consumed by fire, even if enemies had been allowed to set fire to them—for brass could not have melted so quickly. But since the Prophet had predicted this by God’s command, his prophecy was eventually verified after his death, because the event proved that this came from God. For when the doors were removed, the gates themselves were demolished. It may also have been that Darius set fire to them so that he might more quickly destroy the gates and the towers, which were very high, as well as the walls.

He afterwards adds, Labor shall the people in vain, and the nations in the fire; they shall be wearied. This passage is commonly explained as if the Prophet had said that when the walls of Babylon had begun to burn and the gates to be consumed with fire, there would be no remedy, even though the Babylonians might greatly weary and fatigue themselves in attempting to quench the fire.

But this interpretation seems forced and unnatural. I therefore understand the words, though future in form, as referring to the past. The walls of Babylon had not been erected without great labor, and a vast number of men had been hired: some to bring bitumen, others to heap up earth, and still others to make bricks. The Prophet in this place intimates that all this labor would be in vain, precisely because it was spent for the fire—meaning that whatever those who had been either hired for wages or forced by authority to erect the walls did, was labor for the fire; that is, they labored so that their work might eventually be consumed by fire.

This seems to me to be the Prophet's true meaning. He then says that the people had labored in vain, or for nothing. And why? Because they labored for the fire. In my view, the second clause explains the first.