John Calvin Commentary Numbers 8:21

"And the Levites purified themselves from sin, and they washed their clothes: and Aaron offered them for a wave-offering before Jehovah; and Aaron made atonement for them to cleanse them." — Numbers 8:21 (ASV)

And the Levites were purified, and they washed their clothes. We have already spoken of the washing, for since it was required of all private individuals, it would be much less allowable for the Levites to handle the sacred things unless they were first purified.

But what follows concerning their presentation by the hands of Aaron was a symbolic foreshadowing of the truth, which finally shone out at the coming of Christ.

For it had been predicted by the Prophets long ago that, in the renewal of the Church, those who had until then been only part of the multitude would become Levites.

Therefore, by this figure, God would declare that none even of His elect servants would be approved and accepted by Him unless sanctified by the one Priest.⁴¹⁵ For that reason, an atonement is joined with their offering, so that the Levites might be pure.

⁴¹⁵ Addition in Fr., “Auquel ceste dignita a este commise;” to whom this dignity has been entrusted. “Auquel ceste dignita a este commise;” to whom this dignity has been entrusted.