The Essence of God Never Appeared in Itself. Divine Appearances to the Fathers Wrought by the Ministry of Angels. An Objection Drawn from the Mode of Speech Removed. That the Appearing of God to Abraham Himself, Just as that to Moses, Was Wrought by Angels. The Same Thing is Proved by the Law Being Given to Moses by Angels. What Has Been Said in This Book, and What Remains to Be Said in the Next
Wherefore the substance, or, if it is better so to say, the essence of God, wherein we understand, in proportion to our measure, in however small a degree, the Father, the Son, and the Holy Spirit, since it is in no way changeable, can in no way in its proper self be visible.
22. It is manifest, accordingly, that all those appearances to the fathers, when God was presented to them according to His own dispensation, suitable to the times, were wrought through the creature. And if we cannot discern in what manner He wrought them by ministry of angels, yet we say that they were wrought by angels; but not from our own power of discernment, lest we should seem to any one to be wise beyond our measure, whereas we are wise so as to think soberly, as God has dealt to us the measure of faith; and we believe, and therefore speak. For the authority is extant of the divine Scriptures, from which our reason ought not to turn aside; nor by leaving the solid support of the divine utterance, to fall headlong over the precipice of its own surmisings, in matters wherein neither the perceptions of the body rule, nor the clear reason of the truth shines forth. Now, certainly, it is written most clearly in the Epistle to the Hebrews, when the dispensation of the New Testament was to be distinguished from the dispensation of the Old, according to the fitness of ages and of times, that not only those visible things, but also the word itself, was wrought by angels. For it is said thus: “But to which of the angels said He at any time, Sit on my right hand, until I make your enemies your footstool? Are they not all ministering spirits, sent forth to minister for them who shall be heirs of salvation?” Whence it appears that all those things were not only wrought by angels, but wrought also on our account, that is, on account of the people of God, to whom is promised the inheritance of eternal life. As it is written also to the Corinthians, “Now all these things happened unto them in a figure: and they are written for our admonition, upon whom the ends of the world have come.” And then, demonstrating by plain consequence that as at that time the word was spoken by the angels, so now by the Son; “Therefore,” he says, “we ought to give the more earnest heed to the things which we have heard, lest at any time we should let them slip. For if the word spoken by angels was steadfast, and every transgression and disobedience received a just recompense of reward; how shall we escape, if we neglect so great salvation?” And then, as though you asked, What salvation?— in order to show that he is now speaking of the New Testament, that is, of the word which was spoken not by angels, but by the Lord, he says, “Which at the first began to be spoken by the Lord, and was confirmed unto us by them that heard Him; God also bearing them witness, both with signs and wonders, and with various miracles, and gifts of the Holy Ghost, according to His own will.”
23. But some one may say, Why then is it written, “The Lord said to Moses;” and not, rather, The angel said to Moses? Because, when the crier proclaims the words of the judge, it is not usually written in the record, so and so the crier said, but so and so the judge. In like manner also, when the holy prophet speaks, although we say, The prophet said, we mean nothing else to be understood than that the Lord said; and if we were to say, The Lord said, we should not put the prophet aside, but only intimate who spoke by him. And, indeed, these Scriptures often reveal the angel to be the Lord, of whose speaking it is from time to time said, “the Lord said,” as we have shown already. But on account of those who, since the Scripture in that place specifies an angel, will have the Son of God Himself and in Himself to be understood, because He is called an angel by the prophet, as announcing the will of His Father and of Himself; I have therefore thought fit to produce a plainer testimony from this epistle, where it is not said by an angel, but “by angels.”
24. For Stephen, too, in the Acts of the Apostles, relates these things in that manner in which they are also written in the Old Testament: “Men, brethren, and fathers, hearken,” he says; “The God of glory appeared unto our father Abraham, when he was in Mesopotamia.” But lest any one should think that the God of glory appeared then to the eyes of any mortal in that which He is in Himself, he goes on to say that an angel appeared to Moses. “Then fled Moses,” he says, “at that saying, and was a stranger in the land of Midian, where he begot two sons. And when forty years were expired, there appeared to him in the wilderness of mount Sinai an angel of the Lord in a flame of fire in a bush. When Moses saw it, he wondered at the sight: and as he drew near to behold it, the voice of the Lord came unto him, saying, I am the God of your fathers, the God of Abraham, and the God of Isaac, and the God of Jacob. Then Moses trembled, and dared not behold. Then said the Lord to him, Put off your shoes from your feet,” etc. Here, certainly, he speaks both of angel and of Lord; and of the same as the God of Abraham, and the God of Isaac, and the God of Jacob; as is written in Genesis.
Source: On the Holy Trinity (New Advent)