Numbers 8:19

8:19 I have given the Levites as a gift to Aaron and his sons from among the Israelites, to do the work for the Israelites in the tent of meeting, and to make atonement for the Israelites, so there will be no plague among the Israelites when the Israelites come near the sanctuary.” 1 

1 sn The firstborn were those that were essentially redeemed from death in Egypt when the blood was put on the doors. So in the very real sense they belonged to God (Exod 13:2,12). The firstborn was one who stood in special relationship to the father, being the successive offspring. Here, the Levites would stand in for the firstborn in that special role and special relationship. God also made it clear that the nation of Israel was his firstborn son (Exod 4:22-23), and so they stood in that relationship before all the nations. The tribe of Reuben was to have been the firstborn tribe, but in view of the presumptuous attempt to take over the leadership through pagan methods (Gen 35:22; 49:3-4), was passed over. The tribes of Levi and Simeon were also put down for their ancestors’ activities, but sanctuary service was still given to Levi.