"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
The firstborn were, as has been well said, in the most direct sense the beneficiaries of the death of the (Passover) lamb, and thus in a special way the Lord's possession. They must be given to Him: in the case of animals this was done by death; and in the case of human firstborn, it was done by the substitution of the Levites, on a one for one basis, and - for those who could not be accounted for in this way (see 46 the 273 who were in excess of the number of the Levites) by purchase (47ff), in that they were redeemed by silver, the price being paid in ransom to Aaron and his sons.
But why Levi, rather than another tribe? One possible reason may be found in a consideration of an incident recorded in Exodus 32:2629. Levi had been most zealous for the hon- our of the Lord at the time of the worship of the golden calf. When Moses stood in the gate of the camp and uttered his famous challenge, 'Who is on the Lord's side ...? it is recorded that all the sons of Levi rose magnificently to the challenge and came out decisively on the Lord's side. Here, at a critical moment in the history of the people of God, and at the point of challenge and destiny, they gathered themselves to Moses. May not this be the special reason why God chose them? They had been put to the test, and proved, in much the same way as Caleb and Joshua were later tried and proved, by the way they reacted on their return with the spies from the foray in the Promised Land (Numbers 12-13), when they said, 'We are well able to go in and possess the land'. In both cases, they 'made their future' in terms of qualifying themselves for the assumption of responsibility in the work of God in days that were to come.