What was the sin of Nadab and Abihu? The text of Leviticus 10:1-7 is ultimately unclear about this. One Pentateuch scholar aptly calls this an instance of “intentional ambiguity” on the part of the storyteller/author (see Schnittjer, 99, 324, 413-414). So perhaps we will never know the answer for sure. Nevertheless, many people have contemplated this question, and there are many suggestions out there. How do we evaluate the relative merits of these suggestions? Is there a way to distinguish the plausible theories from the implausible ones? I think so.
The literary genre of ritual—which dominates the book of Leviticus—needs to be interpreted according to ritual categories of thinking. Over the years, a number of scholars have come to realize that the biblical ritual literature is driven by the sacred categories of space, status and time (see, for example, Frank Gorman’s Ideology of Ritual). That is to say, the prescriptions of Leviticus are about making sure the right people (=status) are in the right places at the right times. When these categories are properly maintained, then God’s presence is preserved in the midst of His people and equilibrium is established and enjoyed in the cosmos (see also John Walton’s “Equilibrium”). When these categories are ignored or disturbed, chaos may threaten to jeopardize the presence of God and people may die (cf., Exod 33:3-5; Num 16:35; 2 Sam 6:6-8). But how do these matters specifically apply to the strange (zarah; “foreign” or “unauthorized”) fire of Nadab and Abihu?
Perhaps the “strangeness” of the fire is not so much about the fire itself or the coals themselves. Instead, the offense is more likely related to (one of) the above ritual categories. For example, “strange” might be understood as untimely. That is, Aaron’s sons performed the ritual at the wrong time, thereby upsetting the equilibrium and endangering the Israelites (cf., Lev 16:2; see Rabbi Jeremiah, cited in Milgrom, 634). Or perhaps “strange” is related to status. That is, only the high priest was supposed to do that particular ritual, and the two sons upset equilibrium by usurping Aaron’s role (cf., Lev 16:2; see Harrison, 109; Hartley, 131, 133). Or perhaps “strange” is related to space. That is, Nadab and Abihu may have brought the fire beyond its proper zone (cf., Lev 16:1-2; see Rabbi Jeremiah, cited in Milgrom, 633), or perhaps they brought the fire from a zone outside of the sacred enclosure (cf., Lev 16:12-13; see Gorman, 50, 65; Milgrom, 598, 634). Either way, the fire would be "strange" because it was out of place, thus upsetting equilibrium and bringing danger to the community.
These three proposals are, in my estimation, the best kinds of theories. Interpretive certainty will always elude us because the text is ultimately ambiguous about their specific sin. However, these proposals are all plausible ways of explaining the nature of their disobedience (i.e., going against the command of YHWH; see Lev 10:1) because they are in keeping with the ritual categories of thinking that were likely assumed by the ancient Israelites. I even wonder if the specific sin in Lev 10:1 might be some combination of the above three proposals. In fact, the sin of Nadab and Abihu is referenced again in Leviticus 16:1-2 where it serves as a preface for the rituals of the Day of Atonement on which matters of space, status and time are intricately combined.
The incident in Lev 10:1-2 is not unlike the story in Acts 5:1-11 where Ananias and Sapphira also disrupted the equilibrium of the newly inaugurated sacred space of the church by lying to the Holy Spirit. God indeed revealed his holiness in both of these inaugural events and sent a clear message about the deadly serious importance of maintaining purity by obedience to God’s commands (cf., Lev 10:3; Acts 5:5, 11). Perhaps this is also the kind of thing the Apostle Paul is explaining when he says, “If anyone destroys God’s temple, God will destroy him” (1 Cor 3:17).