Charles Ellicott Commentary Jeremiah 4:2

"and thou shalt swear, As Jehovah liveth, in truth, in justice, and in righteousness; and the nations shall bless themselves in him, and in him shall they glory." — Jeremiah 4:2 (ASV)

And you shall swear. —The conditions are continued: If you will swear by the living Jehovah [“the Lord liveth” being the accepted formula for an oath], in truth, in judgment, and in righteousness.

And the nations shall bless themselves in Him. —This forms the completion of the sentence. If the conditions of a true repentance are fulfilled by Israel, then the outlying Gentile nations shall bless themselves in Jehovah— i.e., shall acknowledge Him and adore Him, be blessed by Him.