‘And they offered unauthorized fire before the Lord, contrary to his command.’ Leviticus 10:1b
This passage is not about the meaning of the burnt offering itself – about this, read Leviticus 1:1-17 – but it is about the service of the priests at the burnt offering. The burnt offering was to be burnt on the altar, day and night, an aroma pleasing to the Lord (Leviticus 1:9). The fire – once kindled during the dedication of the tabernacle - must be kept burning on the altar continuously (6:13) and the priests were obliged to care for it.
But when Aaron and his sons were consecrated as priests (Leviticus 9) and the glory of the Lord appeared to all the people, then two sons of Aaron became so excited that they took their censers, put fire in them and added incense to burn this before the Lord. Out of the joy of their hearts, driven by great zeal, they came up with this. But it was contrary to God’s command! This was unauthorized fire, not fire kindled by command of God, and to God this is detestable (Leviticus 10:1-5). God has no need of fire we kindle on our own, He does not ask for human enthusiasm and zeal of the flesh either.
At first, there was great joy when God Himself sent fire from heaven, which consumed the burnt offering and the fat portions on the altar. And when all the people saw it, they shouted for joy and fell facedown (Leviticus 9:24). But Nadab and Abihu extinguished the heavenly joy by serving God in a willful way.
At Pentecost God revealed His glory by descending with fire from heaven (Acts 2:1-13).
If we understand our priestly mission, the fire of Pentecost will keep on burning, but all our own enthusiasm and the contribution of unauthorized fire will ignite God’s wrath and consume us, just like the sons of Aaron.