Leviticus 10:20

Nadab and Abihu took their censers (machtah, Exo 25:38), and having put fire in them, placed incense thereon, and brought strange fire before Jehovah, which He had not commanded them. It is not very clear what the offence of which they were guilty actually was. The majority of expositors suppose the sin to have consisted in the fact, that they did not take the fire for the incense from the altar-fire. But this had not yet been commanded by God; and in fact it is never commanded at all, except with regard to the incense-offering, with which the high priest entered the most holy place on the day of atonement (Lev 16:12), though we may certainly infer from this, that it was also the rule for the daily incense-offering. By the fire which they offered before Jehovah, we are no doubt to understand the firing of the incense-offering. This might be called "strange fire" if it was not offered in the manner prescribed in the law, just as in Exo 30:9 incense not prepared according to the direction of God is called "strange incense." The supposition that they presented an incense-offering that was not commanded in the law, and apart from the time of the morning and evening sacrifice, and that this constituted their sin, is supported by the time at which their illegal act took place. It is perfectly obvious from Lev 10:12. and 16ff. that it occurred in the interval between the sacrificial transaction in ch. 9 and the sacrificial meal which followed it, and therefore upon the day of their inauguration. For in Lev 10:12 Moses commands Aaron and his remaining sons Eleazar and Ithamar to eat the meat-offering that was left from the firings of Jehovah, and inquires in Lev 10:16 for the goat of the sin-offering, which the priests were to have eaten in a holy place. Knobel's opinion is not…

The story of this chapter is as sad an interruption to the institutions of the levitical law as that of the golden calf was to the account of the erecting of the tabernacle. Here is, I. The sin and death of Nadab and Abihu, the sons of Aaron (Lev 10:1, Lev 10:2). II. The quieting of Aaron under this sore affliction (Lev 10:3). III. Orders given and observed about the funeral and mourning (Lev 10:4-7). IV. A command to the priests not to drink wine when they went in to minister (Lev 10:8-11). V. The care Moses took that they should go on with their work, notwithstanding the agitation produced by this event (Lev 10:12, etc.).