John Calvin Commentary Jeremiah 49:4

"Wherefore gloriest thou in the valleys, thy flowing valley, O backsliding daughter? that trusted in her treasures, [saying], Who shall come unto me?" — Jeremiah 49:4 (ASV)

Because people's minds continually waver, as they do not adequately consider the infinite power of God, the Prophet, to remove all obstacles that might have made his prophecy doubtful, now declares that the Ammonites gloried in vain in their valleys. Some understand "valleys" to mean a fertile, well-watered land.

But the Prophet, in my opinion, refers instead to fortified places. He then says that they in vain gloried in their deep valleys; since they were surrounded by mountains, they thought they could not be approached. He derides this vain confidence: Why, he says, do you glory in your valleys, or depths?

Your valley has flowed down. By saying that the valley, or depth, had flowed down, he alludes to its situation. For when anyone considers a region situated among mountains, the land appears to be flowing, like a river gliding between its banks. It is then a striking allusion to a deep place when he says that the valley flowed down. This was the same as if he had said, “Your depth has vanished,” or, “It will not be such a protection to you as you think.”

But the meaning is that even though the Ammonites, trusting in their defenses, ignored all enemy attacks, they would still be exposed to plunder. For their mountains and valleys would be of no avail to them, despite the opinion they held that they were so fortified that they could not be attacked.

He calls Ammon a rebellious, or a backsliding, daughter, though he mentions no specific details. But Ezekiel, Amos, and Zephaniah—these three—clearly show why God was so severe towards the Ammonites (Ezekiel 25; Amos 1:13; Zephaniah 2:9): it was because they had uttered blasphemies against Him and His people, rejoiced over the miseries and calamities of the chosen people, and plundered them when they saw them overcome by their enemies. For these reasons, then, our Prophet now calls them a rebellious people: they had proudly exalted themselves against God and exercised cruel tyranny towards the miserable Israelites, who were still, as has been stated, connected with them by blood.

Who trusts in her secret places, or hidden places—rendered by some as “in her treasures.” But since אצר (atser) means to hide, the reference is, in my opinion, to strongholds. For the Prophet, in the next words, explains himself: Who can come to me? It appears, then, that the Ammonites thought themselves so secure because they were not exposed to their enemies but were protected by their mountains, as if they were in hiding places.

This boasting sufficiently shows that they did not trust so much in their treasures as in their hidden places, because they dwelt in recesses. The meaning is that even though the Ammonites boasted that they were beyond the reach of danger, God would still become the avenger of the cruelty they had exercised towards their relatives, the Israelites.