John Calvin Commentary Jeremiah 31:37

"Thus saith Jehovah: If heaven above can be measured, and the foundations of the earth searched out beneath, then will I also cast off all the seed of Israel for all that they have done, saith Jehovah." — Jeremiah 31:37 (ASV)

He confirms the same thing by another comparison: that it would be impossible for God wholly to forget His covenant, but that He would again gather His people. Exile might indeed appear as a permanent death; and thus the truth of God might have been brought to nothing, and the covenant could not have been made void without giving the people a sort of right to complain that they had been deceived.

For we know that, though a condition was added to the covenant, it was not founded on the integrity of men; and therefore it is said that God is not a liar, though all the Jews were faithless (Romans 3:3, 4). The Prophet then teaches us here that, though God had severely punished the sins of the people and had resolved to punish them in the future, even to destroy their city, there would still be a place for mercy after the people had been chastised.

He had said before that God’s covenant with Abraham's children could no more fail than the laws of nature. He now says that if anyone could measure the heaven and investigate the foundations of the earth—that is, penetrate into the very center of the earth—then, He says, I will reject the seed of Israel. But God brings before us these strange and impossible things so that we may know that He will finally be reconciled to His people after having justly punished them. This promise could not have afforded any consolation to hypocrites, because God does not include the whole seed of Abraham, but says that He would not allow the whole seed of Abraham to perish, for some remnant would continue, according to what is said by Isaiah:

“Though thy people were as the sand of the sea,
a remnant shall be saved.” (Isaiah 10:22)

God then does not here affirm that He would be merciful to all, but that some would still remain, so that the name of the people would continue immortal. In short, He promises that the Church would be saved, but that their number would be small.

We now perceive the Prophet's design: He doubtless had regard for the faithful, who might have been overwhelmed with despair on seeing themselves driven far away from their own country and having no hope of a return. Then he testifies that God had such care for the safety of the faithful that He would gather the scattered seed.

But we must bear in mind what we have said: that this promise is to be confined to the elect alone, for they alone were capable of receiving this favor. As for the unbelieving, who were perverse in their wickedness, God might have wholly cut them off and yet save the remnants of grace.

Now there is no need here to enter into a subtle discussion about whether the center of the earth can be found. Philosophers indeed present some probable reasons regarding the extent of the heavens, and they also conjecture the dimension of the earth. But the Prophet’s purpose was to declare, according to the common and popular way of speaking, that God’s mercy toward the children of Abraham would be perpetual and immeasurable, like the immensity of the earth and the heavens, which exceeds human comprehension.

He adds, On account of all the things which they have done; that is, though they have deserved to die eternally a hundred times, I will still have regard for My covenant and My mercy. The Prophet then intentionally sets before us here the sins of the people, so that we may know that God’s mercy would be very great, seeing that the whole mass of so many evils would not hinder God from forgiving them. This is the reason why He says, on account of all the things which they have done.