John Calvin Commentary Numbers 8:5

"And Jehovah spake unto Moses, saying," — Numbers 8:5 (ASV)

And the Lord spake unto Moses. Although the Levites were not allowed to go into the sanctuary, but were only the priests’ ministers and primarily employed in menial duties, yet, since they carried the tabernacle and the sacred vessels, prepared the sacrifices, and took away the ashes and other refuse from the altar, God willed that they be consecrated to Himself by a solemn rite.

For as all Israel, in relation to the Gentiles, was God’s peculiar people, so the house of Levi was chosen from among the people themselves to be His own property, as is stated here.

But, to prevent them from claiming for themselves more than was right, God forestalled their presumption in these ways:

1. First, by postponing their consecration for some time;
2. Secondly, by directing that they should not be initiated by Moses, but by Aaron; and
3. Thirdly, by appointing a different ceremony for it.

For, if they had been initiated at the same time as the priests, on this pretext they might have contended for equality with them. Therefore, although the priests were already set apart from the common people, the Levites still remained unconsecrated (privati), so that they might learn to reverence the priestly office.

Furthermore, since, if they had also been dedicated by Moses, there was a danger of their being puffed up with pride against all others, Aaron was appointed to preside over their consecration, so that they would humbly submit to his authority.

Also, since they were purified only with water and sacrifice, and without the addition of anointing, the difference in the external rite reminded them that their degree of honor was not similar or the same.