John Calvin Commentary Jeremiah 48:42

"And Moab shall be destroyed from being a people, because he hath magnified himself against Jehovah." — Jeremiah 48:42 (ASV)

He repeats what we have previously observed: that the calamity of Moab would be a just reward for his pride and indeed his sacrilege. The Prophet then says that though God’s vengeance might seem extremely grievous, yet it was most just, because the Moabites had not only been cruel toward their neighbors but also reproachful toward God. Here, then, he condemns them first for cruelty, and then for their impious pride, because they exalted themselves against God.

But we must bear in mind the reason noted before; for the Moabites did not openly boast that they were equal or superior to God, but when they raised their crests against God’s people, they became contemptuous toward God himself, who had promised to be the protector and Father of his people. Since the Moabites thus despised the protection and promise of God, they are here justly condemned by the Prophet: that they exalted themselves against God. This ought to be carefully noted, so that we may not do any wrong to the godly, for God will eventually show that he is injured in them. Great consolation can also be drawn from this: that all who persecute us are waging war against God, and that all who injure us act sacrilegiously toward him. For the Prophet has previously explained how the Moabites boasted against God, specifically because they regarded the children of Israel with derision.