Malachi 3:3-4

3, 4. The first task of the Lord will be to purge the priests, that he may have once more (compare Malachi 2:5-6) a pure priesthood. Here as everywhere in the book the priests stand in the foreground.

He shall sit As a judge upon the judgment seat; and yet his primary purpose is not to condemn, but to sift the good from the worthless, though in the process of sifting the dross will be burned.

Sons of Levi The priests, who were the descendants of Levi (see on Malachi 2:4-5, and references there).

Purge them From all sins and impurities that have made them unfit to represent the people before Jehovah (compare Malachi 1:9; Malachi 2:8-9). This done, they may again serve before the altar.

In righteousness Not only in outward conformity to the law, but in a right state of heart, mind, and life. Offered by the regenerated priests, sacrifice will again be acceptable to Jehovah, and not, as now, abominable (Malachi 1:9; Malachi 2:8).

Days of old,… former years When the faithful Levi (Malachi 2:4-6) ministered in the sanctuary.