Exod 19:18

“But the sound of that voice and the corporeal appearance of the dove and the "parted tongues as it were of fire that sat upon every one of them," like those terrible manifestations that happened on Mt. Sinai and that pillar of cloud by day and of fire by night, were performed and carried out as figurative acts. Now in these matters special care must be taken lest anyone believe that the nature of God, either the Father, the Son or the Holy Spirit, is subject to change or transformation. And let no one be troubled because sometimes the sign receives the name of the thing signified. Thus the Holy Spirit is said to have descended on Christ in the corporeal appearance, as it were, of a dove and to have remained upon him. Thus also the rock is called Christ because it signifies Christ.”

“What shall I say about this except that no one is so insane as to believe that the smoke, the fire, the cloud and the darkness and everything else of a similar nature are the substance of either the Word and the Wisdom of God which Christ is or of the Holy Spirit. For not even the Arians have dared to say this about God the Father. Therefore all those things were produced by a creature serving its Creator and were manifested by a suitable dispensation to the human senses.… Perhaps carnal thoughts will suggest that the cloud was certainly seen by the people but that within the cloud Moses saw the Son of God with his bodily eyes, whom the raving heretics will have to be seen in his own substance, because it was said, "Moses entered the cloud where God was."”