Numbers chapters 3 and 4 are simply about numbering the Levites and prescribing their work for moving the tabernacle and executing ritual worship. Note the rationale for setting aside the Levites for preserving tabernacle worship in Numbers 3:11-13:
“The Lord also said to Moses, 12 “I have taken the Levites from among the Israelites in place of the first male offspring of every Israelite woman. The Levites are mine, 13 for all the firstborn are mine. When I struck down all the firstborn in Egypt, I set apart for myself every firstborn in Israel, whether human or animal. They are to be mine. I am the Lord.”
On my first reading, I misunderstood; I thought God was intentionally mistaking their birth order, calling the Levites “the firstborn” to make a point of their new status. Of course, the firstborn of Israel (i.e., Jacob) was Reuben, and Levi was the third-born (Genesis 29:31-35). Upon re-reading, I realized the Levites are now standing “in place of the first male offspring of every Israelite woman”. This isn’t about the birth order of the sons of Israel, but about the Levites replacing all the firstborn children spared at the Passover when all the Egyptian firstborn were killed. Those spared firstborn sons should have served as God’s dedicated priests, having received the honor of being set aside for such a great task. Yet because the Levites refused to worship the golden calf with the rest of Israel (presumably including many firstborn sons that were spared), the Levites enjoy unique access into the presence of God and service at His tabernacle. This is not the first time, nor will it be the last time in scripture that later-born children and those outside a select group are elevated over the firstborn and elite. Through the Levites, God conveys that He does not bestow the best of His blessings through typical means like birth order, but through grace to those who are righteous. Even so, the Levites themselves are not immune to losing their status, as we will see in the days of the Kings. May this humble us to remain faithful to God, for it pleases God to vindicate His faithful.