4. Mose 3:11

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 Exo 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 (Num 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 (Exo 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 Persia…

This chapter and the next are concerning the tribe of Levi, which was to be mustered and marshalled by itself, and not in common with the other tribes, intimating the particular honour put upon them and the particular duty and service required from them. The Levites are in this chapter considered, I. As attendants on, and assistants to, the priests in the temple-service. And so we have an account, 1. Of the priests themselves (Num 3:1-4) and their work (Num 3:10). 2. Of the gift of the Levites to them (Num 3:5-9), in order to which they are mustered (Num 3:14-16), and the sum of them taken (Num 3:39). Each particular family of them is mustered, has its place assigned and its charge, the Gershonites (Num 3:17-26), the Kohathites (Num 3:27-32), the Merarites (Num 3:33-39). II. As equivalents for the first-born (Num 3:11-13). 1. The first-born are numbered, and the Levites taken instead of them, as far as the number of the Levites went (Num 3:40-45). 2. What first-born there were more than the Levites were redeemed (Num 3:46, etc.).

3:11-13 The firstborn sons . . . of Israel belonged to God, having been spared when the Egyptian firstborn sons died (3:13; see Exod 11–12). The Levites were chosen . . . to serve as substitutes in their place.