22 It is the Angel of God Who appeared in the fire from the bush; and it is God Who spoke from the bush amid the fire. He is manifested as Angel; that is His office, not His nature. The name which expresses His nature is given you as God; for the Angel of God is God. But perhaps He is not true God. Is the God of Abraham, then, the God of Isaac, the God of Jacob, not true God? For the Angel Who speaks from the bush is their God eternally. And, lest you insinuate that the name is His only by adoption, it is the absolute God Who speaks to Moses. These are His words:— And the Lord said unto Moses, I Am that I Am; and He said, Thus shall you say unto the children of Israel, He that is has sent me unto you. God's discourse began as the speech of the Angel, in order to reveal the mystery of human salvation in the Son. Next He appears as the God of Abraham, and the God of Isaac, and the God of Jacob, that we may know the name which is His by nature. Finally it is the God that is Who sends Moses to Israel, that we may have full assurance that in the absolute sense He is God.
Source: On the Trinity (New Advent)