John Calvin on Genesis 18:1

1. And the Lord appeared unto him It is uncertain whether Moses says, that God afterwards appeared again unto Abraham; or whether, reverting to the previous history, he here introduces other circumstances, which he had not before mentioned. I prefer, however, the former of these interpretations; namely, that God confirmed the mind of his servant witha new vision; just as the faith of the saints requires, at intervals, renewed assistance. It is also possible that the promise was repeated for the sake of Sarah. What shall we say, if in this manner, he chose to do honor to the greatness of his grace? For the promise concerning Isaac, from whom, at length, redemption and salvation should shine forth to the world, cannot be extolled in terms adequate to its dignity. Whichever of these views be taken, we perceive that there was sufficient reason why Isaac was again promised. Concerning the word Mamre we have spoken in the thirteenth chapter Genesis 13:1. Probably a grove of oaks was in that place, and Abraham dwelt there, on account of the convenience of the situation.