John Calvin on Jeremiah 33:21

Hence he says, There shall be abolished my covenant with David my servant, that he should not be my son, etc. He repeats what he had said, even that it could not be but that the posterity of David should obtain the kingdom, which we know has been fulfilled in Christ. The throne of David he now calls what he had named before as the throne of the house of Israel; but he means the same thing. It is called the throne of the house of Israel, because the king and the people are relatively connected, and also because the posterity of David ruled for the public good, not for their own sake.

He adds, and with the Levites, the priests, my ministers He had called David his servant, he now calls the Levites his ministers. The word שרת sheret, is commonly known, and is used often by Moses, when speaking of the Levitical priesthood. Its meaning is to serve. He adds —

Jeremiah 33:22

22. As the host of heaven cannot be numbered, neither the sand of the sea measured: so will I multiply the seed of David my servant, and the Levites that minister unto me.

22. Sicut non numeratur exercitus coelorum, et non mensuratur (in futureo tempore ponuntur hoec verba, sed ira resolvi debent, sicutli non potest numerari exercitus coelorum, et non petest modum habere; מדד significat metiri; mensurare non est Latinum verbum, qualiquam cogimur uti; sicut ergo non mensuratur) arena maris; sic multiplicabo semen Davidis servi mei, et Levitarum ministrorum meorum (non dicit sicuti proximo versu, uno verbo, משרתי, sed dicit משרתי אותי, hoc est, qui ministrant mihi, sed idem est sensus)