23 After God had often spoken with Abraham, Sarah was moved to wrath against Hagar, being jealous that she, the mistress, was barren, while her handmaid had conceived a son. Then, when Hagar had departed from her sight, the Spirit speaks thus concerning her, And the angel of the Lord said unto Hagar, Return to your mistress, and submit yourself under her hands. And the angel of the Lord said unto her, I will multiply your seed exceedingly, and it shall not be numbered for multitude, and again, And she called the Name of the Lord that spoke with her, You are God, Who hast seen me. It is the Angel of God Who speaks, and speaks of things far beyond the powers which a messenger, for that is the meaning of the word, could have.
He says, I will multiply your seed exceedingly, and it shall not be numbered for multitude. The power of multiplying nations lies outside the ministry of an angel. Yet what says the Scripture of Him Who is called the Angel of God, yet speaks words which belong to God alone? And she called the Name of the Lord that spoke with her, You are God, Who hast seen me. First He is the Angel of God; then He is the Lord, for She called the Name of the Lord; then, thirdly, He is God, for You are God, Who hast seen me.
He Who is called the Angel of God is also Lord and God. The Son of God is also, according to the prophet, the Angel of great counsel. To discriminate clearly between the Persons, He is called the Angel of God; He Who is God from God is also the Angel of God, but, that He may have the honour which is His due, He is entitled also Lord and God.
Source: On the Trinity (New Advent)