Ref: (Lev. 16: 11-17, 2, 34), (Lev. 10: 1-11), (Exo. 30: 1-9).
God gave Moses various commandments and ordinances of the tabernacle, including burning incense upon the incense altar, and burning incense in censer. As per (Exodus. 30: 7-9) daily three times only the burning of the incense upon the incense altar should be done by the priest and any other burning incense upon it would be strange incense. 1. Early in the morning, 2. When Aaron was dressing the lamp, 3. At evening when Aaron was lighting the lamp. Regarding burning incense in censer must be carried out only by the high priest once in a year in the most holy place, with the blood of bullock as sin offering for the high priest and his family, and also with the blood of kid of goat for the sins of the congregation of Israel, and he had to wear the holy garments of linen clothes (Lev. 16:1-34) Apart from this statute, if incense was burnt in censer it would be strange fire which was not commanded by God. Nadab and Abihu took their censers and filled with fire (from the Altar of burnt offering) and put the sweet incense on the fire in the censer and offered to the Lord in the most holy place which was not commanded by God and it became strange fire which brought a judgment upon the two priests and they died by the fire came out from the most holy place (Lev. 10: 1, 2).
Some Bible readers think that the two priests had brought strange fire from outside, and not from the altar of burnt offering! But by careful reading of the word of God, we can understand that such sorrowful instance took place during the consecration of Aaron as high priest and his four sons as priests, for a period of seven days (Lev.8: 1-36). Everything went well during the seven days and their consecration was over, and they remained in the tabernacle for all the seven days. On the next day or on the eighth day Moses asked Aaron to offer certain sacrifices and asked the congregation to bring certain sacrifices to be offered and then they will see the glory of God (Lev. 9: 1-6). As advised by Moses, Aaron and his sons and the congregation of Israel brought their sacrifices and they offered them as sacrifices and Aaron lifted his hands and blessed the congregation, and Moses and Aaron entered the tabernacle and came out and both blessed the congregation, and the glory of God appeared, and fire from the presence of God came and devoured the sacrifices and fat on the altar of burnt offering, and the people fell on their faces and glorified God (Lev. 9: 22-24). So, bringing fire from outside was not possible and not necessary, as the fire is readily available on the altar of burnt offering. But there was a little breathing period for the priests on the eighth day and they might had some wine for relaxing (may be or may not be). This could have made them to offer incense offering by their censers and they offered strange fire which was not commanded by God, and it caused the tragedy (Lev. 10: 1,2). It is suspected that the two priests might be intoxicated as the Lord spoke to Aaron after their death, “Do not drink wine or strong drink nor thou and thy sons with thee when ye go into the tabernacle of congregation lest ye die: it shall be a statute forever throughout your generations:” (Lev. 10: 8,9)
Also Moses told Aaron “This is that the LORD spake saying I will be sanctified by them that come nigh me, and before all the people I will be glorified “(Lev. 10:3). Here let us co relate an occasion where the name of the LORD was not sanctified by Moses and Aaron, in which instead of speaking to the Rock, Moses smote the rock, and God’s name was not sanctified before the people, and a chastisement came on Moses and Aaron that they could not enter the promised land (Num. 20:12). But the Name of the LORD was sanctified in the people though they strove the LORD, and the Rock yielded water for the people to drink (Lev. 20:13). So we understand that the Name of the LORD was sanctified before all people, when two priests did against the commandment of God and died. Here people feared God, accepting to follow the commandment of God. Regarding Moses, he did not sanctify the Name of the LORD before people and a chastisement came upon him, and the Name of the LORD was glorified by revealing the Truth that the commandment of God must be fulfilled. But regarding the people they strove against God, but God was faithful in supplying water for the thirsty people and the Name of the LORD was sanctified in the people by God (Num. 20: 13). So the Name of the LORD is sanctified in three things such as1. When God’s commandment is trespassed a punishment/ chastisement is revealed, and the Name of the LORD is sanctified, by bringing a fear of God to abide in His commandments, and there by glorified. 2. When God’s commandment is fulfilled correctly, the Name of the LORD is sanctified and glorified. 3. When the commandment of God is trespassed, it brings a punishment/chastisement but God is faithful in fulfilling His duty for His people and the Name of the LORD is sanctified in them by God and glorified.
So the strange fire was a fire from altar, but not commanded by God.