Fire runs through Scripture as a symbol of God's presence and holiness — the burning bush, the pillar of fire leading Israel, the flames at Pentecost. It signals that God is near, and that he is holy.
Fire also refines: God is called a consuming fire, and he tests and purifies faith like gold in the furnace, burning away what is worthless and leaving what is precious.
Fire in Scripture
And the angel of the LORD appeared unto him in a flame of fire out of the midst of a bush: and he looked, and, behold, the bush burned with fire, and the bush was not consumed.