וַיֵּרָא אֵלָיו יְ׳הוָה
Parashat Vayera begins with ‘א’s appearing to Avraham as three men in the Terebinths of Mamrei in order to give over the information that Sarah will give birth to a son, Yitzhak. The problem this immediately raises is that the visitation in chapter 18 follows close on the heels of the revelation in chapter 17, the main theme of which was the promise of Yitzhak’s birth. If Avraham has already been informed that Yitzhak is going to be born, then why does ‘א need to come announce it a second time? However, while the two passages share many similarities and are, to some degree, dependent upon each other, they also differ in many distinct ways, and close analysis of these differences shows that each of the passages expresses a very different perspective on the birth of Yitzhak and its meaning, both for ‘א and for Avraham.
The differences between Bereishit 17 and Bereishit 18 begin almost immediately. Bereishit 17 contains the mitsvah of Brit Milah, circumcision (17:9-14), that is the sign of the covenant between ‘א and Bnei Yisrael, going all the way back to Avraham, a command that appears nowhere else in the Torah. It also includes the discussion of the covenant, and the fact that Yitzhak, not Yishmael, will be Avraham’s successor in the covenant (17:19-20), none of which is mentioned in Bereishit 18. The message of Yitzhak’s birth is given with a focus specifically on Avraham in chapter 17 (17:16, 19), even though Sarah will be the one pregnant, while in chapter 18 it is said specifically that Sarah will have a son (18:10). At no point in chapter 17 are we told the setting of the prophecy, while in chapter 18 a great deal is made of the fact that it is occurring at the tent, which is mentioned five times. To top it off, the chapters use different names for ‘א throughout, a strong differentiator. The passages are also clearly dependent on each other, in that Avraham is not referred to by name in chapter 18 until verse 6. Up to that point he is simply referred to by the pronoun “him” and it is only because this passage follows chapter 17 that we know who is being referred to. But for the fact that the two passages share the pronouncement of the birth of Yitzhak, they would be two separate texts dealing with very separate stories.
Despite that, the two passages share many similarities. After briefly mentioning Avram’s age, the prophecy in chapter 17 begins with ‘א appears to Avram (17:1), using exactly the same phrase found at the beginning the next chapter. Bereishit 17:17 mentions that Avraham laughed, just as Sarah does in Bereishit 18:12-15. In both passages Avraham is predicted to give birth to great nations (17:16, 18:18). The two chapters seem in many ways to be telling the same story, and therefore we must answer why the same story is repeated twice in the Torah, one after the other.
Each of these stories forms the context for the announcing of Yitzhak’s birth, and there are certain hallmarks of that story, such as the disbelief of an elderly parent-to-be, that occur in each passage. But each of these stories forms a very different context and therefore lends to the birth of Yitzhak a very different significance.
In Bereishit 17, the announcement of the birth is in context of the covenant between ‘א and Avraham. It is for this reason that it includes the discussion of the covenant and the command for its sign, the Brit Milah. Yitzhak is not primarily Avraham and Sarah’s son but ‘א’s next covenantal partner (as opposed to Yishmael), who happens to be born to Avraham and Sarah. This is also why there is an emphasis on Yitzchak being born to Avraham, as opposed to Sarah, for Avraham is ‘א’s current covenantal partner. Thus even Avraham’s place in the story is subordinate to the focus on the covenant and ‘א’s greater plan, rather than being about Avraham himself.
In Bereishit 18, the focus is on Sarah and Avraham as individuals, as an elderly barren couple instead of as the Bearers of the Covenant. Thus, the emphasis is on Sarah having a child at least as much as on Avraham, which fits with the way the emphasis is on Sarah being barren in Bereishit 11:30, as opposed to Avraham. Moreover, where in Bereishit 17 the need for a covenantal partner to succeed Avraham would require Yitzhak’s birth, regardless of Avraham and Sarah’s personal merit, Bereishit 18 puts a strong emphasis on their personal merit, emphasizing the way Avraham rushed to greet the guests (18:2), the way both Sarah and Avraham helped prepare food for the guests (18:4-8), and the fact that Avraham would teach his descendant to follow ‘א’s path, to be righteous and just (18:19). Bereishit 18 depicts the birth of Yitzhak as the granting of a child to a couple who had long been childless and who, both because of who they were and who they would teach him to be, could not be more deserving.
‘א acts on the level of his great historical purposes, where sometimes the personal merit of individuals is ignored for the good of ‘א’s plan. But he also interacts on the subtle, individual, stage, where He exists in relation to individuals, and their merit is the deciding factor. These two approaches each surface at various times throughout Tanakh. The great teshuvah movement of King Yoshiyahu ends tragically without ‘א relenting from his decree, as Yoshiyahu was told would be the case by the prophetess Huldah (Melakhim II 22). By contrast, in Yehezkal 18 it is repeatedly stated that all that matters is a person’s personal merit, the things they have done right and the things they have done wrong. These two approaches are manifest in Bereishit 17 and 18, respectively. Bereishit 17 is all about the fact that ‘א has a plan that involves Avraham and his successor, Yitzhak. Bereishit 18 is all about the fact that Sarah and Avraham personally merit a child.
This differentiation is emphasized through the way that the two chapters use different names for ‘א. Bereishit 17 refers to ‘א as Elokim, the name associated with divine justice, with ‘א’s orderly plan for creation, while Bereishit 18 refers to ‘א as YHVH, the name associated with divine compassion, with ‘א’s intimate involvement with His world.
It is important for us to understand these two ways in which ‘א interacts with the world, for the sake of understanding ‘א and understanding History. But it is more important for the sake of our understanding our place in History and the significance of our own actions. Bereishit 17 asks us to understand that we are players on the historical stage, that we participate in ‘א’s plan and further His goals. On this level, it is not our personal merit that makes us significant, but the fact we are ‘א’s chosen covenantal partners, and the incredible responsibility this asks us to bear. Simultaneously, we are individuals who are asked to be kind and gracious, piously dedicated and personally responsible. Existing on the world stage does not exempt us from our local responsibilities. Participation in the historical covenant does not mean that we can be rude to the man on the street; in fact, quite the opposite. The way justice and righteousness play out on the personal stage bear great significance for the realization of ‘א’s historical goals (Bereishit 18:19).
 The most likely explanation of the nature of the appearance (18:1) seems to be that it is constituted by the visit of the angels. They are constantly referred to as men (18:2, 16, 22, etc.), despite being angels (19:1), as this is how they appeared to Avraham. He only became aware that they were not ordinary men when the primary angel spoke as ‘א (18:13). Such is the explanation of the Rashbam. For more, see R’ Elchanan Samet’s explanation here.
 I owe much of the differentiation between the two passages to R’ Yonatan Grossman’s essay “News of the Birth of Yitzchak.”
 This dependence is picked up the midrash, quoted by Rashi on 18:1, that says ‘א was visiting Avraham when he was sick after the circumcision as the mitsvah of Bikkur Holim, Visiting the Sick. In theory, the visitation by the angels could have come at any time, it did not need to be immediately after the circumcision, but the language highlights the fact that the two passages are in fact highly connected.