And the Lord spake unto Moses. Although the Levites were not allowed to go into the sanctuary, but were only the priests' ministers, and chiefly employed in servile duties, yet, inasmuch as they carried the tabernacle and the sacred vessels, prepared the sacrifices, took away the ashes and other offscourings from the altar, God would have them consecrated to Himself by a solemn rite. For as all Israel, with respect to the Gentiles, was God's peculiar people, so the house of Levi was chosen out of the people itself to be His own property, as it is here said. But, lest they should arrogate to themselves more than was right, God anticipates their presumption: first, by putting off their consecration for some time; secondly, by desiring that they should not be initiated by Moses, but by Aaron; and thirdly, by appointing a different ceremony for it. For, if they had been initiated at the same time as the priests, under this pretext they might have contended to be on an equality with them; therefore, although the priests were already separated from the common people, yet the Levites still remain unconsecrated, (privati,) in order that they may learn to reverence the priestly office. And again, since, if they had been dedicated likewise by Moses, there was a danger of their being puffed up with pride against all others, Aaron is appointed to preside over their consecration, that they may modestly submit themselves to his authority. Since, too, they were only purified by 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.

The Levites could only undertake their duties Numbers. 3; 4 after the formal exchange of the Levites for the first-born Numbers 3:44-51.
The distinction between the "consecration" of the priests Leviticus. 8 and the less solemn "purification" Numbers 8:21 of the Levites is marked. These rites of purification are similar to those incumbent on the priests of Egypt.

And the Lord spake unto Moses,.... the same time the order about lighting the lamps was given; and this section is joined to the former, Aben Ezra thinks, to show that Aaron employed himself in lighting the lamps, but in the rest of the services the Levites helped him, and stood before him to do what he commanded them: they were ordered to be taken and given to Aaron before; and they were numbered, and their services appointed to each division of them; and now they are ordered to be ordained and consecrated to their office:
saying; as follows.

Here we have directions for the solemn ordination of the Levites. All Israel must know that they took not this honour to themselves, but were called of God to it; nor was it enough that they were distinguished from others. All who are employed for God, must be dedicated to him, according to the employment. Christians must be baptized, ministers must be ordained; we must first give ourselves unto the Lord, and then our services. The Levites must be cleansed. They must be clean that bear the vessels of the Lord. Moses must sprinkle the water of purifying upon them. This signifies the application of the blood of Christ to our souls by faith, that we may be fit to serve the living God. God declares his acceptance of them. All who expect to share in the privileges of the tabernacle, must resolve to do the service of the tabernacle. As, on the one hand, none of God's creatures are his necessary servants, he needs not the service of any of them; so none are merely honorary servants, to do nothing. All whom God owns, he employs; angels themselves have their services.

Consecration of the Levites for their service in the sanctuary. - The choice of the Levites for service in the sanctuary, in the place of the first-born of the people generally, has been already noticed in Numbers 3:5., and the duties binding upon them in Numbers 4:4. But before entering upon their duties they were to be consecrated to the work, and then formally handed over to the priests. This consecration is commanded in Numbers 8:7., and is not called קדּשׁ, like the consecration of the priests (Exodus 29:1; Leviticus 8:11), but טהר to cleanse. It consisted in sprinkling them with sin-water, shaving off the whole of the hair from their bodies, and washing their clothes, accompanied by a sacrificial ceremony, by which they were presented symbolically to the Lord as a sacrifice for His service. The first part of this ceremony had reference to outward purification, and represented cleansing from the defilement of sin; hence the performance of it is called התחטּא (to cleanse from sin) in Numbers 8:21. "Sprinkle sin-water upon them." The words are addressed to Moses, who had to officiate at the inauguration of the Levites, as he had already done at that of the priests. "Water of sin" is water having reference to sin, designed to remove it, just as the sacrifice offered for the expiation of sin is called חטּאת (sin) in Leviticus 4:14, etc.; whilst the "water of uncleanness" in Numbers 19:9, Numbers 19:13, signifies water by which uncleanness was removed or wiped away. The nature of this purifying water is not explained, and cannot be determined with any certainty. We find directions for preparing sprinkling water in a peculiar manner, for the purpose of cleansing persons who were cured of leprosy, in Leviticus 14:5., 50ff.; and also for cleansing both persons and houses that had been defiled by a corpse, in Numbers 19:9. Neither of these, however, was applicable to the cleansing of the Levites, as they were both of them composed of significant ingredients, which stood in the closest relation to the special cleansing to be effected by them, and had evidently no adaptation to the purification of the Levites. At the same time, the expression "sin-water" precludes our understanding it to mean simply clean water. So that nothing remains but to regard it as referring to the water in the laver of the sanctuary, which was provided for the purpose of cleansing the priests for the performance of their duties (Exodus 30:18.), and might therefore be regarded by virtue of this as cleansing from sin, and be called "sin-water" in consequence. "And they shall cause the razor to pass over their whole body," i.e., shave off all the hair upon their body, "and wash their clothes, and so cleanse themselves." תּער העביר is to be distinguished from גּלּח. The latter signifies to make balk or shave the hair entirely off, which was required of the leper when he was cleansed (Leviticus 14:8-9); the former signifies merely cutting the hair, which was part of the regular mode of adorning the body. The Levites also were not required to bathe their bodies, as lepers were (Leviticus 13:8-9), and also the priests at their consecration (Leviticus 8:6), because they were not affected with any special uncleanness, and their duties did not require them to touch the most holy instruments of worship. The washing of the clothes, on the other hand, was a thing generally required as a preparation for acts of worship (Genesis 35:2; Exodus 19:10), and was omitted in the case of the consecration of the priests, simply because they received a holy official dress. הטּהרוּ for הטּהרוּ, as in 2-Chronicles 30:18.