And the Lord spake unto Moses,.... Continued to speak unto him, and give him the reason of his appointing the Levites to minister to the priests and serve the tabernacle:
saying: as follows.

I have taken the Levites, &c.--The consecration of this tribe did not originate in the legislative wisdom of Moses, but in the special appointment of God, who chose them as substitutes for the first-born. By an appointment made in memory of the last solemn judgment on Egypt (from which the Israelitish households were miraculously exempt) all the first-born were consecrated to God (Exodus 13:12; Exodus 22:29), who thus, under peculiar circumstances, seemed to adopt the patriarchal usage of appointing the oldest to act as the priest of the family. But the privilege of redemption that was allowed the first-born opened the way for a change; and accordingly, on the full organization of the Mosaic economy, the administration of sacred things formerly committed to the first-born was transferred from them to the Levites, who received that honor partly as a tribute to Moses and Aaron, partly because this tribe had distinguished themselves by their zeal in the affair of the golden calf (Exodus 32:29), and also because, being the smallest of the tribes, they could ill find suitable employment and support in the work. (See on Deuteronomy 33:8). The designation of a special class for the sacred offices of religion was a wise arrangement; for, on their settlement in Canaan, the people would be so occupied that they might not be at leisure to wait on the service of the sanctuary, and sacred things might, from various causes, fall into neglect. But the appointment of an entire tribe to the divine service ensured the regular performance of the rites of religion. The subsequent portion of the chapter relates to the formal substitution of this tribe.
I am the Lord--that is, I decree it to be so; and being possessed of sovereign authority, I expect full obedience.

God appointed the Levites for this service, because He had decided to adopt them as His own in the place of all the first-born of Egypt. When He slew the first-born of Egypt, He sanctified to Himself all the first-born of Israel, of man and beast, for His own possession (see Exodus 13:1-2). By virtue of this sanctification, which was founded upon the adoption of the whole nation as His first-born son, the nation was required to dedicate to Him its first-born sons for service at the sanctuary, and sacrifice all the first-born of its cattle to Him. But now the Levites and their cattle were to be adopted in their place, and the first-born sons of Israel to be released in return (Numbers 3:40.). By this arrangement, through which the care of the service at the sanctuary was transferred to one tribe, which would and should henceforth devote itself with undivided interest to this vocation, not only was a more orderly performance of this service secured, than could have been effected through the first-born of all the tribes; but so far as the whole nation was concerned, the fulfilment of its obligations in relation to this service was undoubtedly facilitated. Moreover, the Levites had proved themselves to be the most suitable of all the tribes for his post, through their firm and faithful defence of the honour of the Lord at the worship of the golden calf (Exodus 32:26.). It is in this spirit, which distinguished the tribe of Levi, that we may undoubtedly discover the reason why they were chosen by God for the service of the sanctuary, and not in the fact that Moses and Aaron belonged to the tribe, and desired to form a hierarchical caste of the members of their own tribe, such as was to be found among other nations: the magi, for example, among the Medes, the Chaldeans among the Persians, and the Brahmins among the Indians. יהוה אני לי, "to Me, to Me, Jehovah" (Numbers 3:13, Numbers 3:41, and Numbers 3:45; cf. Ges. 121, 3).