Matthew Henry Commentary Jeremiah 12:14-17

"Thus saith Jehovah against all mine evil neighbors, that touch the inheritance which I have caused my people Israel to inherit: behold, I will pluck them up from off their land, and will pluck up the house of Judah from among them. And it shall come to pass, after that I have plucked them up, I will return and have compassion on them; and I will bring them again, every man to his heritage, and every man to his land. And it shall come to pass, if they will diligently learn the ways of my people, to swear by my name, As Jehovah liveth; even as they taught my people to swear by Baal; then shall they be built up in the midst of my people. But if they will not hear, then will I pluck up that nation, plucking up and destroying it, saith Jehovah." — Jeremiah 12:14-17 (ASV)

The Lord would plead the cause of his people against their evil neighbors. Yet he would afterwards show mercy to those nations, when they learn true religion.

This seems to look forward to the times when the fullness of the Gentiles will come in. Those who desire to cast their lot with God's people, and an ultimate destiny like theirs, must learn their ways and walk in them.