John Calvin on Numbers 18:8

8. And the Lord spake unto Aaron. He now proceeds to state more fully what he had been lately adverting to, as to the rights of the priests with respect to the sacred oblations. We must, however, remember the contrast, which I spoke of, between the priests of the higher order and the Levites; for, whilst the family of Aaron is invested with peculiar honors, the other families of the tribe of Levi are abased. God, then, assigns to the priests alone all the offerings, in which was the greater consecration, called “the holy of holinesses.” A. V. “The most holy things:” “the holy of holies:” Ainsworth, whose note is, “Heb. ‘of the holiness of holinesses,’ i.e., of the most holy things; which the Greek translateth ‘of the hallowed (or sanctified) holy things.’ Some oblations in the sanctuary are called holy, and, by the Hebrew doctors, light holy things, some holy of holies, i.e., most holy things,” etc. An exception will afterwards appear; viz., that the whole was to be deposited, by way of honor, with the priests, out of which they were to pay a part to the Levites, who were performing their office in the service of the sanctuary. He tells them that this privilege is given them “by reason of the anointing,” lest the priests should pride themselves or magnify themselves on this score; for God’s gratuitous liberality ought to instruct us in modesty and humility. It is by this argument that Paul corrects and represses all vain boasting: “Why dost thou glory, as if thou hadst not received it?” (1 Corinthians 4:7.) Now, the sons of Aaron had obtained their anointing by no other right, than that God had been pleased to elect them to it. This is also indicated by their privilege being spoken of as “a gift:” but God thus more expressly commends His grace, for He makes mention of His gift for another reason, i.e., that none should enter into any dispute or controversy with the priests on this point.