malachi 3:3 And he shall sit as a refiner and purifier of silver: and he shall purify the sons of Levi, and purge them as gold and silver, that they may offer unto the LORD an offering in righteousness. - Free Bible Online

      3. sit--The purifier sits before the crucible, fixing his eye on the metal, and taking care that the fire be not too hot, and keeping the metal in, only until he knows the dross to be completely removed by his seeing his own image reflected (Ro 8:29) in the glowing mass. So the Lord in the case of His elect (Job 23:10; Ps 66:10; Pr 17:3; Isa 48:10; Heb 12:10; 1Pe 1:7). He will sit down to the work, not perfunctorily, but with patient love and unflinching justice. The Angel of the Covenant, as in leading His people out of Egypt by the pillar of cloud and fire, has an aspect of terror to His foes, of love to His friends. The same separating process goes on in the world as in each Christian. When the godly are completely separated from the ungodly, the world will end. When the dross is taken from the gold of the Christian, he will be for ever delivered from the furnace of trial. The purer the gold, the hotter the fire now; the whiter the garment, the harder the washing [MOORE].
      purify . . . sons of Levi--of the sins specified above. The very Levites, the ministers of God, then needed cleansing, so universal was the depravity.
      that they may offer . . . in righteousness--as originally (Mal 2:6), not as latterly (Mal 1:7-14). So believers, the spiritual priesthood (1Pe 2:5).

JFB.