On The Holy Trinity by St. Augustine
Chapter 14.--Of the Appearance in the Pillar of Cloud and of Fire.
24. Also in the going forth of the children of Israel from Egypt it is written, |And the Lord went before them, by day in a pillar of cloud to lead them the way, and by night in a pillar of fire. He took not away the pillar of the cloud by day, nor the pillar of fire by night, from before the people.| Who here, too, would doubt that God appeared to the eyes of mortal men by the corporeal creature made subject to Him, and not by His own substance? But it is not similarly apparent whether the Father, or the Son, or the Holy Spirit, or the Trinity itself, the one God. Nor is this distinguished there either, in my judgment, where it is written, |The glory of the Lord appeared in the cloud, and the Lord spake unto Moses, saying, I have heard the murmurings of the children of Israel,| etc.