Leviticus 9:24
And there came a fire out from before the LORD, and consumed upon the altar the burnt offering and the fat: which when all the people saw, they shouted, and fell on their faces. And there came a fire out from before the LORD, and consumed upon the altar the burnt offering and the fat: which when all the people saw, they shouted, and fell on their faces.
The fire that came out from before the Lord and consumed the burnt offering and the fat upon the altar signified God's acceptance of the sacrifice. The offering He thus accepted was one made as He had commanded. That consuming fire also revealed God's glory and was a sign of His glory and favor toward the people. When the people saw it they responded with awe, praise and adoration, shouting and falling on their faces before Him.
The fire on the altar was sacred fire kindled by God Himself from heaven, from before the Lord. That divine fire was to be kept burning continually in the sanctuary. It was to be kept burning day and night and never allowed to go out. God commanded that this sacred fire be used for burning incense, for the fire put upon the censer. It was also to be used for all sacrifice, and no sacrifice was acceptable to God unless salted and seasoned with this divine fire.
The sacred fire was to be distinguished from common or ordinary fire. God had forbidden the use of common fire in His service, so using ordinary fire instead violated His express command. Nadab and Abihu's strange fire stood in opposition to this divinely kindled fire, for they sinned by using common fire for incense instead of the sacred fire. They did so with minds clouded by wine, disregarding God's requirement. Fire from the Lord then devoured them for this act.
The divine fire represented the communication between God and man opened through Jesus Christ alone. While the people of God prayed without, the incense kindled by this holy fire arose before God mingled with their prayers as an emblem of the mediation of Christ. Any common fire used in His service was strange fire that insulted the infinite God and was punishable with death.