The centerpiece of Yom ha-Kippurim, the most singular and pivotal day in the Jewish calendar (referred to thrice in the Torah as "achat ba-shanah"), is the elaborate avodah that enables the kohen gadol to enter into the innermost sanctum (lifnai ve-lifnim) of the beit ha-mikdash. Even in the aftermath of the Temple's destruction, the elaborate description and ecstatic depiction of the avodah dominates tefillat mussaf, mesmerizing all those who contemplate and reflect upon it. Chazzarat ha-shatz of mussaf on Yom Kippur is an inimitable experience, though it is barely "u-neshalmah parim sefateinu" relative to the drama that unfolded in the actual avodah.
The avodah is exotic, unique, and profound in its elaborate shifting between kohen gadol wardrobes (5 tevilot, and ten kidushin), in its range of interspersed korbonot, in the drama of the two parallel and opposing seirim (lifnai ve-lifnim and le-azazel), as well as other singular features. However, it is particularly atypical in projecting a single actor, the kohen gadol, who not only presides but almost completely dominates this intricately complex halakhic protocol that is vital to every member of Klal Yisrael and to the collective of Am Yisrael (Rambam, Hilchot Teshuvah 2:6, "zeman teshuvah le-kol le-yachid u-lerabim, ve-hu keitz mechilah u-selichah le-Yisrael"). It is striking and atypical that such a crucial and urgent dimension of halakhic life is undertaken in a manner that renders Klal Yisrael observers-spectators. Yahadut fundamentally rejects anything that even approximates vicarious attornment, demanding participation and responsibility. Why, then, is the most crucial of all religious experiences the exclusive domain of a single figure, the kohen gadol. Even the modicum of wider communal participation, also emulated by the tzibbur in the mussaf prayers – "ve-kol ha-am ba-azarah ke-shehayu shomein et Hashem...koreim u-mishtachavim ve-noflim al peneihem" - is triggered by and is a response to the actions of the kohen gadol.
This phenomenon contrasts sharply even with other avodah protocols which tend to be inclusive at least of multiple kohanim. The institutions of payasim, mishmarot, batei av etc. reflect an effort to widen priestly participation. On Yom Kippur, however, the kohen gadol's exclusive role extends not only to the singular aspects of avodat Yom Kippur, but even to the daily or generic services - temidim, musafim, as well (see Yoma 32b etc.). While there are fundamental debates among the rishonim (Baal ha-maor and Milhcamot on Yoma 26a, Ritva 12b, Rambam, Hilchot Avoda Yom Hakippurim 1:2, etc.) regarding the precise scope of the kohen gadol's expanded responsibility, the principle is endorsed by all and formulated as an important dimension of the kohen gadol's prominence and indispensability.
Moreover, aside from his daily korban minchat chavitin, the kohen gadol's primary mikdash obligation and role is the once-yearly avodah. The Talmud establishes that there can only be a single qualified kohen gadol for this unique function on this singular day (see also Rashi, Megillah 9b). The fact that he leaves his home to begin a week-long preparation for this avodah is consistent with this theme. Extraordinary steps (that defy the routine rules of safek and chazakah- preparing a replacement, a replacement wife, undergoing extra purity measures etc.) are taken to preclude any mishap that might sideline him or preclude him from his destiny to preside over the avodat Yom Hakippurim. It is possible that numerous norms that regulate the kohen gadol's life and status year-round stem from the anticipation of his Yom Kippur mission that defines and qualifies his very persona! [See Rav Soloveitchiks' explanation of Rambam, Hilchot Issurei Biah 17:13, Kelei Hamikdash 5:10 - the prohibition against multiple spouses.]
The most striking evidence of the intricate bond between the kohen gadol and Yom Kippur is found in the protocol of the avodah itself, as well as in its presentation in parshat Acharei Mot. The Torah introduces avodat Yom Hakippurim by focusing on the circumstances and methodology in which Aharon, and his kohen gadol successors would warrant entry into the kodesh ha-kodoshim. We encounter "bezot yavo Aharon el ha-Kodesh" many verses before the Torah finally elaborates the concept of Yom Kippur! Moreover, what enables the national korbonot - the seirim etc.- that achieve the collective kaparah for Klal Yisrael are the personal korbonot - the par and ayi l - of the kohen gadol which precede them. The Yom Kippur protocol calls for three viduyim and ten shemot Hashem, two thirds of which stem from the personal par of the kohen gadol, the blood of which is ultimately actually merged with the communal seir ha-penimi when they are sprinkled on the mizbeach ha-ketoret! It is fascinating that the Talmud (and commentaries) debate the exact status of these offerings which had to be personally financed by the kohen gadol, but which play such a central and indispensable role in the Yom Kippur process. [See Yoma 6b-8a, 49b-51b. The issues that reflect the status of these korbonot (korban yachid, ztibur, shutafim) - temurah, hutrah be-tzibur, chataot hameitot etc. are complex and subject to numerous interpretations. I hope to elaborate these in a different context.] Numerous other indications cement the impression that the kohen gadol is more than merely the appropriate, even the exclusive functionary of the avodah.
Evidently, the avodat Yom Hakippurim requires the kohen gadol because Yom Kippur itself is a day that both enhances and is enhanced and elevated by the ideal personal of the kohen gadol. The kohen gadol's role and the norms that govern his conduct and standards accentuate the interlocking themes of kulo la-Hashem, total devotion and loyalty of Hashem's service, as well as an absolute commitment to the collective of Klal Yisrael. The kohen gadol even sets aside personal grief and bereavement in order to uphold the principles of Divine and national service. He is makriv onen, and abstains from tumat meit even for close family members. Only a meit mitzvah, perhaps because that is a Klal Yisrael obligation, justifies his ritual impurity. While the Rambam rules that regular kohanim may not wear their priestly garb when they are not in service, he excludes kohen gadol from this injunction (Rambam Hilchot Klei ha-Mikdash 8:12, Hilchot Kilayim 10:32 - see Ra'avad and Radvaz ad loc). Radvaz links this distinction to the fact that the kohen gadol is enjoined from disrupting his mikdash duties even for family bereavement - "min ha-mikdash lo yeitzei". The kohen gadol is existentially always connected the avodat Hashem, as his idealistic persona embodies kulo la-Hashem and absolute devotion to Klal Yisrael. The Rambam rules (Hilchot Klei ha-Mikdash 5:7- absent a known source!) that the kohen gadol should live in Yerushalayim! Perhaps this view is consistent with his celebrated position (Rambam Peirush Hamishnayot Sukkah and Rosh Hashanah, and reflected also in Hilchot Beit haBechirah chapter 6) that Yerushalyim has the legal status of mikdash, as well. Moreover, Yerushalyim (ir she-chubrah yachdav, shelo nit'chalek le-shevatim...) represents the locus of collective Klal Yisrael! Even when the kohen gadol leaves the "mikdash" he remains ensconced in the mikdash region!
During the musaf services, we focus on the personal fate, success, and persona of the kohen gadol. Certainly this emphasis is related to his high stakes Yom Kippur mission representing Klal Yisrael, but the jubilant "mareh kohen" which accentuates his radiance and other esthetic facets seems to transcend the mission. Indeed, the yom tov that his success occasioned was also a celebration of his institutional persona, an appropriate expression not only on his most high profile day, but on the day that magnificently manifests the values he personified.
Yom Kippur is most emphatically not a manifestation of vicarious atonement, a concept that is completely foreign to Yahadut. Rather, it constitutes a day of pure loyalty and commitment, the day of the avodah lifnai velifnim, the day of the persona of the kohen gadol. It is a profound appreciation-celebration of the converging themes of the exclusive institutional persona devoted kulo la-Hashem on the day, achat ba-shanah, in which through fasting, kedushat ha-yom, and the intricate, ambitious avodah, all of Klal Yisrael reaffirms that very capacity.