Ambrose of Milan : And Isaiah shows that the Holy Spirit is not only light but also fire, saying, "And the light of Israel shall be for 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 customarily pointed out or seen in the appearance of fire: "For our God is a consuming fire," as Moses said. — On the Holy Spirit 1.14