Albert Barnes Commentary Jeremiah 12:16

"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." — Jeremiah 12:16 (ASV)

The accomplishment of this blessing depends on both Judah and the Gentiles reversing their past conduct. Then the believing Gentile will be admitted into the fold of the true Israel (because it is spiritual)—Christ’s Church.