For the first and only time in Leviticus, we are introduced to Aaron’s two oldest sons Nadab and Abihu (cf. Ex. 6:23). From Exodus 28 we learn that they must have participated in the ordination process of chs. 8 & 9. These sons experienced God’s presence in the ratification of the covenant in Ex. 24:1, 9.
They offered “unauthorized fire” — the phrase is used two other times but they all refer back to this incident (Num. 3:4; 26:61). Specific incense ingredients were prescribed and perhaps they offered something different into the fire? Some argue this should have been offered by the high priest so they blatantly usurped this role (cf. TOTC). What is clear is that they did what they wanted and not what God prescribed.
Aaron cannot complain even though his sons were destroyed because God acted justly as Moses said, “Among those who are near me I will be sanctified, and before all the people I will be glorified.” (v. 3) They were not permitted to grieve (vv. 6, 7) because they have been consecrated — other people can grieve for them. They were also not to drink while they performed their duties (vv. 8-11).
In vv. 16-20, Aaron’s two sons did not eat the meat of the sin offering which raised Moses’ ire and concern. Aaron’s plea is that on account of what happened, they felt ashamed to eat or they were simply afraid — it was a dangerous situation. This reason seems to have met with Moses’ approval — at least they were not careless but fearful.
Worshipping God must never be gone in a cavalier manner. Being on this side of the cross does not diminish the seriousness of worship. In the New Testament, Annanias and Saphira were struck dead for lying against God. In Corinth, some died because of the way they treated each other and the Lord’s Supper. The writer of Hebrew says, “let us offer to God acceptable worship, with reverence and awe, or our God is a consuming fire.” (Heb. 12:28, 29) Through Christ we offer our spiritual worship with reverence and awe. Worship in the New Covenant must never be goofy, irreverent, or casual.