John Calvin Commentary Jeremiah 51:12

"Set up a standard against the walls of Babylon, make the watch strong, set the watchmen, prepare the ambushes; for Jehovah hath both purposed and done that which he spake concerning the inhabitants of Babylon." — Jeremiah 51:12 (ASV)

These words seem to have been addressed to the Chaldeans rather than to the Medes or the Persians, as some interpret them. This interpretation is favored by the context, for as he first commands them to raise a standard on the walls, he then adds, Increase the watch, which refers to the citizens of Babylon. And then he says, set the watchmen. All this cannot apply to the Persians and the Medes but must be referred to the besieged, as it is most suitable for them.

Therefore, I do not doubt that the Prophet here describes, with a taunt, all the efforts the Chaldeans would make to defend their city. For not only those who attack a city raise a standard, but also those who are besieged do so as a sign of confidence, to show that they possess sufficient courage to check their enemies and to sustain all their attacks.

It was then the Prophet's design to show that however strenuously the Chaldeans might defend themselves, all their exertions would be in vain, because God would destroy the city without effort.

He says, Raise, the banner on the walls of Babylon, and strengthen, or increase, the watch; and afterwards, set watchmen, so that everyone might watch with more care than usual. He says at last, set in order the ambushes. "When all things have been tried by you, your labor will be without any advantage, for the Lord hath spoken." When the particle גם, gam, is repeated, it ought to be rendered as and so—for as the Lord has thought, so will He do what He has said, etc.

He says again that God had thought, lest the faithful should imagine that He heedlessly issues threats. For this thought often occurs to the mind: that God terrifies without accomplishing anything.

Hence the Prophet, so that he might more fully confirm his prophecy, says that the matter had been meditated upon by God. We said yesterday that God does not deliberate with Himself as humans do. But since we cannot otherwise understand the certainty and unchangeableness of His secret counsel, nor form an idea of the validity of His decrees, the word thought is mentioned.

In short, the Prophet means that he brought forth nothing but what God had decreed. For words are often heedlessly uttered, and reality and words are not always connected. But Jeremiah testifies that he had taken what he announced from the hidden and immutable counsel of God. Then he adds, what He has spoken or said; and this refers to His doctrine or His prediction.