26 What blind faithlessness it is, what dulness of an unbelieving heart, what headstrong impiety, to abide in ignorance of all this, or else to know and yet neglect it! Assuredly it is written for the very purpose that error or oblivion may not hinder the recognition of the truth. If, as we shall prove, it is impossible to escape knowledge of the facts, then it must be nothing less than blasphemy to deny them. This record begins with the speech of the Angel to Hagar, His promise to multiply Ishmael into a great nation and to give him a countless offspring.
She listens, and by her confession reveals that He is Lord and God. The story begins with His appearance as the Angel of God; at its termination He stands confessed as God Himself. Thus He Who, while He executes the ministry of declaring the great counsel is God's Angel, is Himself in name and nature God. The name corresponds to the nature; the nature is not falsified to make it conform to the name. Again, God speaks to Abraham of this same matter; he is told that Ishmael has already received a blessing, and shall be increased into a nation; I have blessed him, God says.
This is no change from the Person indicated before; He shows that it was He Who had already given the blessing. The Scripture has obviously been consistent throughout in its progress from mystery to clear revelation; it began with the Angel of God, and proceeds to reveal that it was God Himself Who had spoken in this same matter.
Source: On the Trinity (New Advent)