Numbers 8:21

21.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, much less would it be allowable for the Levites to handle the sacred things, unless they were first purified. But what follows as to their presentation by the hands of Aaron, was a shadowing forth by symbol of the truth, which at length shone out at the coming of Christ; for it had been of old predicted by the Prophets, that, in the renovation of the Church, those who had hitherto been but of the multitude should become Levites. Therefore, by this figure, God would declare that none even of His elect servants would be approved of and accepted by Him, unless sanctified by the one Priest. (415) And thence an atonement is joined with their offering, in order that the Levites might be pure.