Numbers 3:11

“And the Lord spake unto Moses, etc.] 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 (Exo 13:12; Exo 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…”

“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.”

“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…”