5. sons of Levi--namely, those alone who belonged to the family of Aaron, to whom the priesthood was restricted. Tithes originally paid to the whole tribe of Levi, became at length attached to the priesthood.
      according to the law--sanctioned by Jehovah (Heb 9:19).
      of their brethren--with whom, in point of natural descent, they are on a level.
      though, &c.--Though thus on a level by common descent from Abraham, they yet pay tithe to the Levites, whose brethren they are. Now the Levites are subordinate to the priests; and these again to Abraham, their common progenitor; and Abraham to Melchisedec. "How great" (Heb 7:4) then, must this Melchisedec be in respect to his priesthood, as compared with the Levitical, though the latter received tithes! and now unspeakably great must "the Son of God" be, to whom, as the sacerdotal archetype (in God's purpose), Melchisedec was made like! Thus compare the "consider," Heb 7:4, in the case of Melchisedec, the type, with the "consider" (Greek, "contemplate attentively," see on Heb 3:1, a stronger word than here) in the case of Christ, the archetype.

JFB.