John Calvin Commentary Numbers 18:25

"And Jehovah spake unto Moses, saying," — Numbers 18:25 (ASV)

And the Lord spoke to Moses. This is another kind of tithe, that is, a hundredth part of the whole produce, which the Levites paid to the priests. Some consider a third kind; but I have given my reasons why I do not agree with this opinion. Surely it is not probable that in the same year double tithes were exacted and paid. Let this twofold division, therefore, be enough for us.

A larger portion was given to the priests, not only as an honorable distinction, but because greater holiness and integrity in using them was expected from them, and also so that they might meet many particular burdens. To prevent the Levites from being too base and stingy, God declares that their theft would be no less wicked if they dealt dishonestly toward the priests than if the people were to withhold any part of their own just share. For this is the purpose of the words: that the tithe of the tithes, which they are commanded to pay, should be as if they paid it from the threshing-floor and the wine-press (verse 27), as if to say that they were no more exempted from the second tithes than the people from the first.

The precept is then still further extended, namely, that they should offer a part of all the offerings. Thirdly, sincere liberality is urged upon them, so that they should not set aside as the priests’ portion anything that was lean or in poor condition, or in any respect of inferior quality, but that they should rather offer whatever was choicest. For this is what is meant by the word חלב, cheleb,²²⁴ which some translate adeps; the word pinguedo seemed more suitable, which, however, contains a metaphor.

²²⁴ Ver. 30. . A.V. “the best the best thereof.” Margin, “Heb., fat.” According to Facciolati, fat.” According to Facciolati, adeps, means liquid fat which does not readily become hard; and means liquid fat which does not readily become hard; and pinguedo, simply fat, “an oily substance in animals and other things."simply fat, “an oily substance in animals and other things."