27 The course of the Divine narrative is accompanied by a progressive development of doctrine. In the passage which we have discussed God speaks to Abraham, and promises that Sarah shall bear a son. Afterwards three men stand by him; he worships One and acknowledges Him as Lord. After this worship and acknowledgment by Abraham, the One promises that He will return hereafter at the same season, and that then Sarah shall have her son. This One again is seen by Abraham in the guise of a man, and salutes him with the same promise.
The change is one of name only; Abraham's acknowledgment in each case is the same. It was a Man whom he saw, yet Abraham worshipped Him as Lord; he beheld, no doubt, in a mystery the coming Incarnation. Faith so strong has not missed its recognition; the Lord says in the Gospel, Your father Abraham rejoiced to see My day; and he saw it, and was glad. To continue the history; the Man Whom he saw promised that He would return at the same season. Mark the fulfilment of the promise, remembering meanwhile that it was a Man Who made it.
What says the Scripture? And the Lord visited Sarah. So this Man is the Lord, fulfilling His own promise. What follows next? And God did unto Sarah as He had said. The narrative calls His words those of a Man, relates that Sarah was visited by the Lord, proclaims that the result was the work of God. You are sure that it was a Man who spoke, for Abraham not only heard, but saw Him. Can you be less certain that He was God, when the same Scripture, which had called Him Man, confesses Him God?
For its words are, And Sarah conceived, and bare Abraham a son in his old age, and at the set time of which God had spoken to him. But it was the Man who had promised that He would come. Believe that He was nothing more than man; unless, in fact, He Who came was God and Lord. Connect the incidents. It was, confessedly, the Man who promised that He would come that Sarah might conceive and bear a son. And now accept instruction, and confess the faith; it was the Lord God Who came that she might conceive and bear.
The Man made the promise in the power of God; by the same power God fulfilled the promise. Thus God reveals Himself both in word and deed. Next, two of the three men whom Abraham saw depart; He Who remains behind is Lord and God. And not only Lord and God, but also Judge, for Abraham stood before the Lord and said, In no wise shall Thou do this things, to slay the righteous with the wicked, for then the righteous shall be as the wicked. In no wise will You Who judgest the whole earth, execute this judgment. Thus by all his words Abraham instructs us in that faith, for which he was justified; he recognises the Lord from among the three, he worships Him only, and confesses that He is Lord and Judge.
Source: On the Trinity (New Advent)