And mount Sinai was completely in smoke, because the LORD descended upon it in fire: and the smoke of it ascended as the smoke of a furnace, and the whole mount quaked greatly.
All Commentaries on Exodus 19:18 Go To Exodus 19
Augustine of Hippo
AD 430
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.