Before you read any further, I would like to suggest that you stop now, friend, and read Leviticus chapters 8 and 9--yes both chapters.
Okay, now that you have read these chapters. I want you to key in on the phrase:
"just as the LORD had commanded Moses." In various forms, you'll find the phrase found ten times in the two chapters, wherein God was dedicating the tabernacle.
Doing things right mattered. After offering sacrifices and preparing the tabernacle, Lev. 9:23-24 says:
Moses and Aaron went into the tent of meeting. When they came out and blessed the people, the glory of the LORD appeared to all the people. Then fire came out from before the LORD and consumed the burnt offering and the portions of fat on the altar; and when all the people saw it, they shouted and fell on their faces.
This is the setting for Nadab and Abihu, who had worn the linen and offered the sacrifices "just as the Lord had commanded" for several days.
Their offering of "strange" fire (unauthorized fire in ESV and NIV), was something the Lord had not commanded. They thought that one up on their own and acted on their own initiative.
God's response was to consume them with fire--fire that came from the presence of the Lord.
The Lord said through Moses, ‘Among those who are near me I will be sanctified (treated as holy, NASB), and before all the people I will be glorified.’
God took personal offense at the unauthorized offering. Offering something strange was treating Him as common, not holy. Their acting on of their own initative did not glorify God. They may have had good intentions; but God was not glorified by their doing their own thing.
Some folks who think telling this story amounts to legalism and who have made grace appear to allow presumption (which David calls great transgression in Psalm 19) point to Eleazar and Ithamar, who had lost their brothers.
In their grief, they did not eat the sin offering that day. Should they have? Yes. Did they? No, it burned uneaten. The blood was not brought into the holy place.
When Aaron explained, "When things like these happened to me, if I had eaten a sin offering today, would it have been good in the sight of the LORD?” Aaron realized that his heavy heart would not allow him to eat this offering with a proper heart. He felt it better not to eat than to render to God an inappropriate heart in his eating. This explanation seemed good in Moses' sight. No more was said about it.
God is certainly a God of compassion, mercy and grace; Moses aceepted the explanation. To find God's understanding in a time of great loss is understandable. To knowingly, willfully act without authority is quite another.
Does one example of grace open the door for presumption and disregard? Some make the mistake here of thinking that it is easier to get forgiveness than it is to obey; but such a view may be presumptuous. It is one thing to explain under some extreme circumstances why one cannot fulfill his duty; it is another to presume upon the grace of God.
Nadab and Abihu, Uzzah, Saul, Ahaz, and Uzziah all learned that one has no right to act without authorization.
Jeremiah spoke of some prophets who in their hearts were sure they were speaking for the Lord. "Thus saith the Lord..." was part of their prophecies. The only problem is that the Lord did not say those things. It never entered into God's mind to say those things. They dreamed them up--spoke from their own imagination. "They speak a vision of their own imagination, Not from the mouth of the LORD." “I did not send these prophets, But they ran. I did not speak to them, But they prophesied." “Then as for the prophet or the priest or the people who say, ‘The oracle of the LORD,’ I will bring punishment upon that man and his household." (see Jer. 23:16-40) Acting without authority is dishonoring to God and exceedingly sinful.
Paul noted how some ate the Lord Supper unworthily (1 Cor. 11:23-32), not discerning the Lord's body and thus incurring judgment. Their hearts and minds did not focus on the Lord but on the fusses brewing over the behavior of their brothers. Some were weak and sick, many were spiritually asleep. God wanted right hearts with right action. I would think that it would be better not to partake at all than to partake unworthily--not honoring God and treating Christ as holy. This is more parallel to Aaron, Eleazar and Ithamar than Nadab and Abihu. While Moses accepted Aaron's explanation, neither Moses nor Aaron expected this to be the case the next time they had a sin offering to sacrifice. It is one thing to forgive a diversion over extreme circumstances; it is another to act like God didn't care whether he was obeyed or not. The counsel of presuming on the grace of God couldn't be more destestable.
This is a different thing than presumptuously acting on one's own initiative. The difference is that in Nadab and Abihu's case, they acted without authority and innovated their own offering. They acted without a command at all. In the second situation, the brothers acted out of soorow, respecting the commandment so much that they would not eat with an improper heart.
Loophole religion, presuming we can continue to act where we have no authority, is sand theology. Those who counsel sand theology will see their house stand for a time; but the wind will blow and the rain will come. Their counsel will be so regretted. Sand theology is thinking I can do whatever I please and face no consequences. Sad, sad.
Grace ought to lead us to listen and build on a rock, to love enough to heed. Grace builds faith and careful desire to please--not presumptuous innovation. We must not presume upon grace and try the Lord.