Jamieson, Fausset and Brown on Jeremiah 30:21

21. their nobles--rather, "their Glorious One," or "Leader" (compare Ac 3:15; He 2:10), answering to "their Governor" in the parallel clause.

of themselves--of their own nation, a Jew, not a foreigner; applicable to Zerubbabel, or J. Hyrcanus (hereditary high priest and governor), only as types of Christ (Ge 49:10; Mi 5:2; Ro 9:5), the antitypical "David" (Jr 30:9).

cause him to draw near--as the great Priest (Ex 19:22; Le 21:17), through whom believers also have access to God (He 10:19-22). His priestly and kingly characters are similarly combined (Psa 110:4; Zec 6:13).

who ... engaged ... heart to approach--literally, "pledged his heart," that is, his life; a thing unique; Messiah alone has made His life responsible as the surety (He 7:22; 9:11-15), in order to gain access not only for Himself, but for us to God. Heart is here used for life, to express the courage which it needed to undertake such a tremendous suretyship. The question implies admiration at one being found competent by His twofold nature, as God and man, for the task. Compare the interrogation (Is 63:1-3).