And the light of Israel shall be for a fire, and his Holy One for a flame: and it shall burn and devour his thorns and his briers in one day;
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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"