Numbers 3:12
12 And I, behold, I have taken the Levites from among the children of Israel instead of all the firstborn that openeth the matrix among the children of Israel: therefore the Levites shall be mine;
If God commanded that firstborn sons from all the tribes be given Him, why was the tribe of Levi given instead?
PROBLEM: God had commanded Moses that “all the firstborn of man among your sons you shall redeem” (Exodus 13:13; cf. 22:29). However, this was never done. Instead, one whole tribe was set apart to God for the work of the priesthood (Numbers 3:12).
SOLUTION: The reason for the substitution is given in Numbers 3:12. The Lord declared: “I Myself have taken the Levites from among the children of Israel instead of every firstborn who opens the womb among the children of Israel.” Since they belonged to Him, God had the right to make the substitution.