Nicene and Post-Nicene Fathers, Vol. II:
City of God: Chapter 13

Chapter 13.—Of the Invisible God, Who Has Often Made Himself Visible, Not as He Really Is, But as the Beholders Could Bear the Sight.

Neither need we be surprised that God, invisible as He is, should often have appeared visibly to the patriarchs.  For as the sound which communicates the thought conceived in the silence of the mind is not the thought itself, so the form by which God, invisible in His own nature, became visible, was not God Himself.  Nevertheless it is He Himself who was seen under that form, as that thought itself is heard in the sound of the voice; and the patriarchs recognized that, though the bodily form was not God, they saw the invisible God.  For, though Moses conversed with God, yet he said, “If I have found grace in Thy sight, show me Thyself, that I may see and know Thee.” 406   And as it was fit that the law, which was given, not to one man or a few enlightened men, but to the whole of a populous nation, should be accompanied by awe-inspiring signs, great marvels were wrought, by the ministry of angels, before the people on the mount where the law was being given to them through one man, while the multitude beheld the awful appearances.  For the people of Israel believed Moses, not as the Lacedæmonians believed their Lycurgus, because he had received from Jupiter or Apollo the laws he gave them.  For when the law which enjoined the worship of one God was given to the people, marvellous signs and earthquakes, such as the divine wisdom judged sufficient, were brought about in the sight of all, that they might know that it was the Creator who could thus use creation to promulgate His law.

Footnotes

Ex. 33.13.