Masechet Yoma deals primarily with the avodah on Yom Kippur. The Masechet opens by describing the preparation of the kohen gadol in the days prior to the yom tov. The fifth Mishnah describes how on erev Yom Kippur the elder kohanim would take the kohen gadol to the attic of beit avtinas where they would teach him to perform avodah of burning the ketoret in the kodesh hakodashim. They would then make him swear that he would not diverge from the method he had been instructed, when performing the avodah.1
The Bartenura explains that their concern was that this kohen gadol might be a Tzeduki – a Sadducee – who dismissed the oral tradition. One of the points of debate between the tzedukim and the perushim (those faithful to the oral tradition) was regarding the two handfuls of ketoret that were offered in the kodesh ha'kodashim on Yom Kippur. The Tedukim maintained that the ketoret was placed on the shovel of coals prior to entering the kodesh ha'kodashim. Their position was based on the pasuk – "…for in a cloud I will appear above the kaporet" (Vayikra 16:2). The Chachamim however understood the koteret was burnt inside the kodesh ha'kodashim. This is based on the pasuk, "he shall place the incense on the fire before Hashem…" Since the kohen gadol was the only one to enter the kodesh hakodashim and no one would be there to see how he performed the avodah, it was necessary for them to make the kohen gadol swear that he perform the avodah as they instructed them.
The Tosfot R' Akiva Eiger cites the question of the Pri Chadash. If the kohen gadol was indeed a tzaduki, how was the oath a deterrent? According to the tzedukim, the perushim were in error. The tzeduki might then lie in order to fulfil the mitzvah according to their understanding. To put it differently, for a tzeduk he was being asked to swear to annul a mitzvah, which would not be binding. The Pri Chadash answers that since this oath was well known, it may act as a deterrent for a tzeduki kohen who did not treat oaths lightly.
The Sefat Emet (Yoma 18b) presents a few answers. Firstly, he suggests that since the law that an oath to anul a mitzvah is not binding is based on expositions, the tzedukim may not have drawn the same conclusion. In other words, for them, this oath might be binding and therefore effective to ensure that they perform the avodah as required.
The Sefat Emet also suggest that this oath was broader than the this specific avodah. He notes that the Mishnah suggests that this was the case – "swear that you will not stray from anything that we have told you". A shevua of this nature, that is kollel (encompasses) other things including this mitzvah is indeed binding in its entirety. The Tifferet Yisrael also suggests this answer.
Finally the Sefat Emet explains that even if the tzeduki would consider this case as an oath to annul a mitzvah and not binding, making such an oath would be considered a shevuat shav (an empty oath) and still prohibited. Consequently, requesting the kohen gadol to make the oath would be a good test to see if he was a tzeduki since his refusal would indicate that he perceived the shevua as a shevuat shav.
The Tifferet Yisrael however notes that in the Mishnah description, prior to the oath, they would assert that they were the shluchei (messengers) of the beit din and that he was our messenger. The Tifferet Yisrael suggest therefore that part the oath the he was only acting as kohen gadol on condition that he perform the ketoret as they instructed him. Consequently, irrespective of how the tzedukim understood the process, if the kohen gadol acted out of line, he would not be considered the kohen gadol and his avodah would be invalid.
1 This explanation is according to the Bartenura. The Tifferet Yisrael however explains that it was the Beit Din that made the kohen gadol swear prior to the ziknei kahuna teaching him how to peform the avodah involving the ketoret.
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