The flame of fire is a sign of the holiness of God. When Israel came to Sinai God appeared to them in fire (Exodus 19:18). He is a consuming fire—a jealous God who does not accept idolatry (see Deuteronomy 4:24). He educated his people to have fear for the Lord and keep his commandments (Deuteronomy 4:36). On Pentecost God the Holy Spirit also came with the sign of fire (Acts 2:3).
2 And the angel of the LORD appeared to him in a flame of fire out of the midst of a bush. He looked, and behold, the bush was burning, yet it was not consumed.