Ver. 19.—O Lord, my strength, and my fortress, etc. Jeremiah falls into the tone of the psalmists (Ps. 18:2; 28:8; 59:17). All that is choicest and most permanent in Old Testament religion finds its adequate lyric expression in the Book of Psalms. The Gentiles shall come unto thee. The article, however, is not expressed. “Nations,” i.e. a crowd of peoples, hitherto ignorant of the true God, shall hasten to the scene of Jehovah’s great interposition; they have been convinced by Israel’s unlooked-for restoration of the unique divinity of Jehovah.
Ver. 20.—But the Jews of this generation, in spite of the manifold proofs of the true religion which have been vouchsafed to them, are deserting the real divinity for the unreal. In a tone of surprise the prophet exclaims, Shall a man make gods unto himself, etc.?
Ver. 21.—The final answer of Jehovah. There will be no further grace-time. I will this once cause them to know; rather, I will this time (comp. on ch. 10:18) cause them to acknowledge. The judgment which Jeremiah has had the sad duty of announcing will prove to the blinded Jews that Jehovah alone is true God, alone can strike and heal.
Spence-Jones, H. D. M. (Ed.). (1909). Jeremiah (Vol. 1, p. 398). London; New York: Funk & Wagnalls Company.
Views from an Evangelist 