25 In another instance the Scripture reveals through Abraham that it was God Who spoke. He receives the further promise of a son, Isaac. Afterwards there appear to him three men. Abraham, though he sees three, worships One, and acknowledges Him as Lord. Three were standing before him, Scripture says, but he knew well Which it was that he must worship and confess. There was nothing in outward appearance to distinguish them, but by the eye of faith, the vision of the soul, he knew his Lord.
Then the Scripture goes on, And He said unto him, I will certainly return unto you at this time hereafter, and Sarah your wife shall have a son; and afterwards the Lord said to Him, I will not conceal from Abraham My servant the things that I will do; and again, Moreover the Lord said, The cry of Sodom and Gomorrha is filled up, and their sins are exceeding great. Then after long discourse, which for the sake of brevity shall be omitted, Abraham, distressed at the destruction which awaited the innocent as well as the guilty, said, In no wise will You, Who judgest the earth, execute this judgment.
And the Lord said, If I find in Sodom fifty righteous within the city, then I will spare all the place for their sakes. Afterwards, when the warning to Lot, Abraham's brother, was ended, the Scripture says, And the Lord rained upon Sodom and upon Gomorrha brimstone and fire from the Lord out of heaven; and, after a while, And the Lord visited Sarah as He had said, and did unto Sarah as He had spoken, and Sarah conceived and bare Abraham a son in his old age, at the set time of which God had spoken to him. And afterwards, when the handmaid with her son had been driven from Abraham's house, and was dreading lest her child should die in the wilderness for want of water, the same Scripture says And the Lord God heard the voice of the lad, where he was, and the Angel of God called to Hagar out of heaven, and said unto her, What is it, Hagar?
Fear not, for God has heard the voice of the lad from the place where he is. Arise, and take the lad and hold his hand, for I will make him a great nation</em>.
Source: On the Trinity (New Advent)