In conjunction with the Revelation at Har Sinai, the Torah states: "And against the eminent ones of B'nei Yisroel He did not send His hand; and they saw Hashem, and ate and drank." (Shemos 24:11) Rashi and the Ramban, each invoking various midrashic interpretations, provide vastly different explanations of this mysterious pasuk.

Rashi explains that the pasuk refers to Nadav, Avihu and the Zekeinim (Elders) experiencing a Divine vision at Sinai shortly before the Torah was given, while they ate and drank. Although this was a fatal misdeed, for Nadav, Avihu and the Zekeinim ought not have partaken of food and drink while experiencing a vision of the Shechinah, and they were subject to mortal punishment for such conduct, Hashem held back and did not mete out punishment at the time so as to not disturb the simchah of the Revelation; rather, punishment for this act occurred later, in association with other events. This is Rashi's interpretation of the matter.

The Ramban, on the contrary, explains that the pasuk expresses praise for Nadav, Avihu and the Zekeinim. According to the Ramban, the pasuk describes how Nadav, Avihu and the Zekeinim, due to their merit and elevated status, experienced a special Divine vision at Mattan Torah (the Giving of the Torah), and how they and the rest of the nation adhered to the command (in Shemos 19:24) not to ascend Har Sinai or approach it beyond specific boundaries. As a result of their careful adherence to this command, Hashem did not strike Nadav, Avihu and the Zekeinim when they experienced that special vision at Sinai. The Ramban further explains that the eating and drinking upon this occasion was a mitzvah, as one is required to rejoice and feast upon receiving the Torah, as per Midrash Shir Ha-Shirim Rabbah (1:9).

What is at the crux of this machlokes (dispute) between Rashi and the Ramban?

One approach to explain the machlokes is to take a step back and analyze it in the larger context of the events that the Torah depicts at this juncture. Rashi, adopting one midrashic position, maintains that the episode under discussion occurred just prior to Mattan Torah, as B'nei Yisroel were encamped at Sinai and were spiritually preparing themselves for the Revelation. According to this understanding, Nadav, Avihu and the Zekeinim were guilty of gazing upon the Divine while consuming food and drink, in the context of spiritual preparation for Mattan Torah, whereas they should rather have been in a state of greater humility and trepidation. On the other hand, since the Ramban interprets the events at Sinai in our parshah as having transpired at the conclusion of Ma’amad Har Sinai (the Sinai Experience), the celebration of Nadav, Avihu and the Zekeinim with food and drink was wholly appropriate, as it was an expression of simchah following the receiving of the Torah, rather than in a context of spiritual preparation and expected trepidation prior to Mattan Torah.

However, there is an alternative approach to understanding the machlokes of Rashi and the Ramban.

The Revelation at Sinai constituted a mass geirus (conversion to Judaism) of the entire nation. Whereas prior to Mattan Torah, B'nei Yisroel were a clan or tribe, whose members' identities were defined by their fathers' lineage, stemming from the Avos (Patriarchs), the Revelation infused the entire nation with Kedushas Yisroel  - Jewish spiritual holiness (which is passed on matrilineally), halachically precipitating complete Jewish status. The Gemara (Kerisos 9a) explains that just as our ancestors entered the Covenant (i.e. acceptance of the Torah and conversion to full Jewish status) by undergoing circumcision, immersion in a mikveh, and offering korbonos, so must all future geirus contain these elements. (Circumcision is obviously only pertinent in the case of a male ger, and the offering of korbonos by a ger is only commanded when sacrificial service is in practice.) Although geirus does feature these required procedures, the essence of geirus is (1) one's entering into a special relationship with Hashem ("to be sheltered under the wings of the Shechinah" - Rambam, Hil. Issurei Bi'ah 13:4), and (2) accepting of the Torah.

A critical question is which of these two essential elements constituted the crux of the geirus at Sinai (and serves as the crux of all future conversion to Judaism). Was the Revelation itself, in which B'nei Yisroel encountered the Shechinah and were brought intimately close Hashem's Presence, the defining, core aspect of the geirus, or was the acceptance of the Torah the main facet of the geirus at Sinai?

It would appear that the machlokes of Rashi and the Ramban hinges precisely upon this issue. Rashi would maintain that the Revelation, the experiencing of closeness with the Shechinah and the drawing of B'nei Yisroel intimately close to Hashem's Presence, was the defining aspect of the geirus at Sinai. Thus, feasting during this incredibly solemn and awesome event was to be harshly condemned. On the other hand, the Ramban would maintain that the Mattan Torah aspect of the Sinai experience, the mass acceptance of the Torah, was the central event, and the Revelation was more of contextual import. Hence, since the defining aspect of Ma'amad Har Sinai was Kabbalas Ha-Torah, the formal acceptance of the Torah, celebrating the event with a feast was fully appropriate.

It is interesting that the Rishonim (medieval halachic authorities) differ regarding the requirement for a ger to verbally accept the mitzvos before a beis din at the point of conversion. Tosafos (on Yevamos 45b) and the Rosh (ibid. 4:31) require a formal, verbal acceptance of the mitzvos by every ger in the presence of the officiating beis din. The Shulchan Aruch (Yoreh Deah 268:3) rules this way as well. However, the requirement for such a verbal declaration is absent from the Rambam's codification of the halachos of geirus. (The Rambam clearly views a ger's full commitment to observe the entire Torah as an absolute and indispensable requirement, as per Hil. Issurei Bi'ah ibid., and as per the words of the Gemara itself in Yevamos 47a; v. Sefer Hararei Kedem p. 264 as well as Reshimos Shiurei Rav Yosef Dov Ha-Levi Soloveitchik:Yevamos, pp. 508-509. It is merely the formal verbalization of such that the Rambam does not require.)

It would seem that Tosafos and the Rosh maintain that the essence of geirus is acceptance of the Torah on the part of the ger; hence their ruling that a verbal declaration of acceptance of the mitzvos by every ger in the presence of a beis din is always required, as acceptance of the Torah is the defining facet of geirus. This concept is confirmed by the Ramban's commentary on the Gemara (on Yevamos 45b), where the Ramban rules like Tosafos and the Rosh - precisely in line with the Ramban's position that the essence of the geirus at Sinai was the acceptance of the Torah.

On the other hand, the Rambam would define geirus primarily as entering into a special and close relationship with Hashem, such that the relationship, echoing back to the Revelation and the intimate encounter with the Shechinah at Sinai, is the essence of geirus, and Torah and mitzvos provide the necessary context and parameters of the relationship. This is evidenced by the words of the Rambam (Hil. Issurei Bi'ah ibid.): "And thus it is for perpetuity, when a non-Jew seeks to enter the Bris (Covenant) and to be sheltered under the wings of the Shechinah, and he accepts upon himself the yoke of Torah...." The primary element of geirus, according to the Rambam, is entering into a special relationship with Hashem and drawing intimately close to the Shechinah, with Torah as the defining context. Hence does the Rambam not require a formal verbal declaration by the ger of acceptance of the mitzvos, as acceptance of Torah and mitzvos serves as the necessary condition and parameters of geirus but is not its essence, according to the Rambam.