3:4 See Leviticus 10:1, 2 for the story of Nadab and Abihu.
3:5-13 At the time of the first Passover, God instructed every Israelite family to dedicate its firstborn son to him (Exodus 13:2). They were set apart to assist Moses and Aaron in ministering to the people. This was only a temporary measure, however. Here God chose all the men from the tribe of Levi to replace the firstborn sons from every Israelite tribe (3:40-51 and 8:16). These men, called Levites, were set apart to care for the Tabernacle and minister to the people. All the priests had to belong to the tribe of Levi, but not all Levites were priests. The Levites were to be 25 years old before entering service. They probably received five years of on-the-job training before being admitted to full service at age 30.
3:10 Aaron and his descendants were appointed to the priesthood. There is a tremendous contrast between the priesthood of Aaron in the Old Testament and the priesthood of Christ in the New Testament. Aaron and his descendants were the only ones who could carry out the duties of the priests and approach God’s dwelling place. Now that Christ is our High Priest—our intermediary with God—anyone who follows him is also called a priest (1 Peter 2:5, 9). Now all Christians may come into God’s presence without fear because God’s own Son encourages his followers to do so. We can put guilt behind us and experience forgiveness when we have a special relationship with God based on what Christ has done for us.