And Nadab and Abihu, the sons of Aaron— The offence of Nadab and Abihu, according to almost all the commentators, was their kindling their censers from strange or common fire; not from the fire which burned always upon the altar of the Lord: (see ch. Leviticus 16:12.) One may conceive, from the prohibition of wine to the priests immediately following this catastrophe, Leviticus 10:9 that the too free use of wine had occasioned them to act thus, contrary to what God had commanded; for though there is no law extant prohibiting the offering of common fire, yet it is not to be supposed that they would have been condemned to death had they not done something which God had expressly forbidden, or omitted what he had expressly commanded. Hence the words, which he commanded them not, are thought to imply an express prohibition; as if it had been said, which he had forbidden. See Jeremiah 32:35. As strange incense, i.e. other incense than God had appointed, is forbidden, Exodus 30:9 so strange fire is implicitly forbidden, ch. Leviticus 6:12 as afterwards God sheweth, ch. Leviticus 16:2. We refer to the reflections at the end of the chapter for a further account of this extraordinary event; which, however, a learned writer, in a distinct treatise on the subject, explains in a very different manner: he makes two objections against the common interpretation, remarking, 1st, That Moses gives to the fire, of which the two sons of Aaron made use, the direct name of fire without any qualification; not calling it strange fire till after he had said that they put incense thereon: so that, considering the mode of expression he uses, it seems as if the fire which Nadab and Abihu employed was not in itself a strange fire, and only became such when they had cast the incense upon it. 2nd, He insists, that the last verse of the foregoing chapter destroys the common interpretation; where it is said, that there came a fire out from before the Lord, and consumed upon the altar the burnt-offering and the sat: which when all the people saw, they shouted, and fell on their faces: to which Moses immediately adds, And Nadab and Abihu, the sons, &c. It seems, therefore, that, as soon as the sacred fire had descended upon the burnt-offering and the fat, in the presence of all the people and in view of Aaron and his sons, then, precisely then, these took each of them his censer, and put fire therein. Now, how does it appear that they could have taken of any other fire upon the spot than that which they attended, and which, it is most probable, God had before expressly commanded them to make use of?
But why, then, should Moses call the fire with which Nadab and Abihu furnished themselves strange? To this the learned divine answers, because they put the incense upon this fire in another manner than that which God had ordained. According to him, the passage should be thus rendered, Then Nadab and Abihu, the sons of Aaron, took each of them his censer, and put [sacred] fire therein, and put incense thereon: thus they offered strange fire before the Lord, which had been forbidden them. There are three considerations, which are advanced to confirm this conjecture: 1st, It is certain that the priests were to kindle incense in the holy place upon the golden altar: Maimonides and several rabbies are express on this head. 2nd, It is evident, from the words of the sacred historian, that Nadab and Abihu put the incense upon the fire of their censers previous to their coming before the Lord: this we are led to conclude from the connexion and construction of the passage. Lastly, it is clear, that the Scripture often gives the epithet strange to that which is improperly joined, or mixed with other things. Thus a false worship offered to the true God is called a strange worship, as a prostitute is denominated a strange woman; (Proverbs 7:5.) and the incense, compounded in another manner than God had appointed, whether as to the quantity or quality of the drugs, is called strange incense: (Exodus 9:34-35.) Here, therefore, it is urged in like manner, the strange fire was sacred fire, rendered strange, or impure, by the association of incense put to it, contrary to the rules, and in contempt of the orders which God had given, though Moses does not mention them. See Theodor. Scheltinga, de fato Nadabi & Abihu; and Chais on the passage.