Nadab and Abihu were killed for this "small" thing in God's eyes. Wow! I have a hard time understanding this. Any thoughts?
In Jewish times, the source of the fire in the sanctuary was from God. Initially, when they first laid the sacrifice as an offering at the alter, fire was literally sent from God and consumed it (Leviticus 9:24). Since then it was to be kept continually burning. (Leviticus 6:12-13).
Thus Nadab and Abihu displayed wanton presumptuousness by lighting fire themselves in the censer (instead using fire from the alter) and offering it to the Lord. As if such a disingenuous act would go unnoticed and accepted by God Himself. They rejected God's mercy for the last time, and were stricken down.
Finally, the smoke of the censer represent the cleansing of the prayers of the believers before it reaches God (Revelation 8:4). For them to use human means at a time when divine fire was available and required was incredibly profane.
Is deliberately disobeying God a small thing?
Leviticus 10:3b (NASB)
“By those who come near Me I will be treated as holy, And before all the people I will be honored.”
Considering these words that Moses attributes to God himself, it might be safe to conclude that the punishment was commensurate with the offense.
“Strange fire” is not defined explicitly, other than the typical meaning of strange or foreign. It was not what God told them to do. Considering the penalty, it was probably a direct violation of what God told them to do, as if they did it in some other manner than what was prescribed by God.
Numbers 15:30-31 (NASB)
“But the person who does anything defiantly , whether he is native or an alien, that one is blaspheming the LORD; and that person shall be cut off from among his people. Because he has despised the word of the LORD and has broken His commandment, that person shall be completely cut off; his guilt will be on him.”
God does make examples of disobedience in the early stages of introducing himself to Israel. Aaron's sons are one example, but there are others. The man who broke the Sabbath in Numbers 15 is another.
Execution and Eternal Condemnation
Do not equate execution with eternal condemnation. These are not necessarily the same. Death comes to all men1, regardless of their piety, and God executing someone need not be any different (in terms of final, eternal judgment) than God allowing them to die any other way.
1 Nearly all men, Elijah and Enoch excepted.