Exodus 19:18 And mount Sinai was altogether on a smoke, because Yhaweh descended upon it in fire: and the smoke thereof ascended as the smoke of a furnace, and the whole mount quaked greatly.
Hebrews 12:29 29 For our Yhaweh is a consuming fire.....
Question; Why was the fire in the altar to burn continuously?
Answer; That the fire in the altar was the burn continuously. Yhaweh wanted a perpetual fire there always.This is mentioned three times in this chapter (verses 9, 12, and 13).
Yhaweh appeared to Moses “in flames of fire from within a bush. Moses saw that though the bush was on fire it did not burn up”
Yahweh chose the appearance of a continuous fire when calling Moses to lead the people out of Egypt to a new land. Later, when God was leading the Israelites out of Egypt, He appeared as a pillar of fire at night.
One reason the ongoing fire was so important is that it was started directly by Yhaweh: “Fire came out from the presence of the LORD and consumed the burnt offering and the fat portions on the altar. And when all the people saw it, they shouted for joy and fell face down.
The fire on the altar, therefore, served as a constant reminder of Yahweh's power. It was a gift from heaven. No other source of fire was acceptable to Yahweh.
This fire also represented Yhaweh's presence. Yahweh is a consuming fire.
New Testament, John the Baptist predicted that the Messiah would baptize with the Spirit and with fire
Matthew 3:11; Luke 3:16.
Fire served as a sign of judgment and refining, but it also reminds us of the Holy Spirit’s coming at Pentecost in the form of “tongues of fire.
The continuously burning, divine fire at the altar of burnt offering helped remind the Israelites of the reality of Yahweh's presence and of their need for Yhaweh. The sacred fire endured throughout the 40 years in the desert and likely beyond that, as tabernacle worship continued until the time of King Solomon and the building of the Jewish temple. When the temple was dedicated, God once again lit the fire on the altar.
2 Chronicles 7:1