"5 And the Lord spoke to Moses, saying, 6 “Bring the tribe of Levi near, and set them before Aaron the priest, that they may minister to him. 7 They shall keep guard over him and over the whole congregation before the tent of meeting, as they minister at the tabernacle. 8 They shall guard all the furnishings of the tent of meeting, and keep guard over the people of Israel as they minister at the tabernacle. 9 And you shall give the Levites to Aaron and his sons; they are wholly given to him from among the people of Israel. 10 And you shall appoint Aaron and his sons, and they shall guard their priesthood. But if any outsider comes near, he shall be put to death.”
11 And the Lord spoke to Moses, saying, 12 “Behold, I have taken the Levites from among the people of Israel instead of every firstborn who opens the womb among the people of Israel. The Levites shall be mine, 13 for all the firstborn are mine. On the day that I struck down all the firstborn in the land of Egypt, I consecrated for my own all the firstborn in Israel, both of man and of beast. They shall be mine: I am the Lord."
Numbers 3:5-13
This section recounts the appointment of the Levites as servants to the priests, and they are placed under Aaron's orders - being dedicated to the service of God as substitutes for the firstborn of the people. The significance of the Lord's claim on the firstborn of Israel, and His choice of the Levites instead of them, is somewhat complex, and we must discuss the substitution of the one for the other before we can draw out the lesson that it undoubtedly contains for us. In Exodus 1 we read how the Lord claimed the firstborn of Israel for Himself, following the Passover in Egypt, when all the firstborn of Egypt were slain by the angel of death. This much is clear and without complication. But now (1113, 44ff) the Lord announces that in- stead of the firstborn He will take the tribe of Levi for Himself. In 40ff we are told how this substitution and changeover took place. The question that arises is: Why this change? And why Levi? Well, the selection of one tribe rather than a heterogeneous mixture of the firstborn from all twelve tribes may in fact have been purely a matter of practical expediency. They would be more easily identifiable and more easily managed than a selection from the twelve tribes could possibly have been. With the Levites, God had, so to speak, a unit ready made, and able to work harmoniously together.