He shall sit. As a judge. The prophet confines himself to the first of the two images presented in the preceding verse. The sons of Levi. Especially the priests, who ought to set an example, and teach holiness and obedience.
Thus judgment should begin at the house of God (Ezekiel 9:6; 1 Peter 4:17). The purifying consists not only in exterminating the evil, but also in correcting and improving all who are not wholly incorrigible.
We may call to mind Christ's purging of the temple, and his denunciations of the teaching body among the Jews, and see herein his way of trying his ministers in all ages, that they may shine like lights in the world, and adorn the doctrine of God in all things.
That they may offer (and they shall be offering) unto the Lord an offering (minchah) in righteousness. The pure sacrifice shall then be offered with a pure heart. As firstfruits of this improved condition, we read in Acts 6:7, "A great company of the priests were obedient to the faith."