John Gill Commentary Jeremiah 33:21

"then may also my covenant be broken with David my servant, that he shall not have a son to reign upon his throne; and with the Levites the priests, my ministers." — Jeremiah 33:21 (ASV)

[Then] may also my covenant be broken with David my servant ,
&c.] A type of the Messiah, with whom the covenant of grace is made, stands fast, and will never be broken; see (Psalms 89:3Psalms 89:28Psalms 89:34) ; for, as the other is impossible, the breaking of the covenant with day and night, or hindering the certain rotation of them; so likewise as impossible is the breaking of the covenant with David concerning the perpetuity of his kingdom in the Messiah: that he should not have a son to reign upon his throne ;
which he has in Christ, and ever will have; for he shall reign for ever and ever: and with the Levites my priests, my ministers ;
of the line of Phinehas, to whom an everlasting priesthood was promised, and which has been fulfilled in Christ, who has an unchangeable priesthood; a priesthood that will never pass from him, and go to another; see (Numbers 25:13) (Hebrews 7:24) .