By Ambrose of Milan (337-397 C.E.)
Our God comes; he does not keep silence; before him is a devouring fire, around him a mighty tempest. — Psalm 50.3
What, then, is that fire? Not certainly one roaring with the burning of the reeds of the woods, but that fire which improves good deeds like gold, and consumes sins like stubble. This is undoubtedly the Holy Spirit, who is called both the fire and light of the countenance of God.
And Isaiah shows that the Holy Spirit is not only light but also fire, saying: “The light of Israel will become a fire.” So the prophets called him a burning fire, because in those three points we see more intensely the majesty of the Godhead; since to sanctify is of the Godhead, to illuminate is the property of fire and light, and the Godhead is wont to be pointed out or seen in the appearance of fire: “For our God is a consuming fire,” as Moses said.
For he himself saw the fire in the bush, and had heard God when the voice from the flame came to him saying: “I am the God of Abraham, and the God of Isaac, and the God of Jacob.” The voice came from the fire, and the voice was in the bush, and the fire did no harm.
For the bush was burning but was not consumed, because in that mystery the Lord was showing that He would come to illuminate the thorns of our body, and not to consume those who were in misery, but to alleviate their misery; who would baptize with the Holy Spirit and with fire, that he might give grace and destroy sin. So in the symbol of fire God keeps his intention.
In Acts, when the Holy Spirit had descended upon the faithful, the appearance of fire was seen: “And suddenly there came from heaven a sound like a mighty rushing wind, and it filled the entire house where they were sitting. And divided tongues as of fire appeared to them and rested on each one of them.”
And as there is a light of the divine countenance, so, too, does fire shine forth from the countenance of God. For the grace of the day of judgment shines beforehand, that forgiveness may follow to reward the service of the saints.
*Abridged and language updated from Book One of On the Holy Spirit.