The Mishna in Masechet Yoma (1:5) tells that on Erev Yom Kippur, the elder kohanim would meet with the kohen gadol, who would be performing the Yom Kippur service the next day, and they would administer an oath. The heretical Sadducee sect, which wielded considerable power during the Second Commonwealth, advocated a different manner of offering the incense on Yom Kippur than that mandated by the rabbinic oral tradition, and so the kohen gadol was made to swear that he would perform this service according to the method espoused by Chazal. The Mishna adds that after the oath was taken, the kohen gadol would cry over having been suspected of heresy, and the elders would likewise cry.
Rav Yehuda Leib Ginsburg, in his Musar Ha-mishna, cites two different reasons for why the elders wept after administering this oath. The Gemara (Yoma 19b) explains that the prohibition of “chosheid bi-ksheirim” – wrongly suspecting an innocent person – is an especially grave matter, and violators are liable to severe punishment. The elders thus wept out of fear that they may have transgressed this prohibition. The Talmud Yerushalmi, however, explains differently, commenting that the elders wept for the very fact that such an oath was necessary. The unfortunate reality that high priests, entrusted with the vitally important responsibility of bringing atonement through the Yom Kippur service, needed to be suspected of heresy, was sufficient cause for wails of anguish.
Rav Ginsburg elaborates further on the Yerushalmi’s remark, suggesting that it perhaps touches upon a broader phenomenon. Namely, the prospect of a kohen gadol deviating from accepted halakhic norms in deference to heretical ideas reflects the universal susceptibility to sin. At least symbolically, it represents the fact that nobody is ever free from the clutches of the yetzer ha-ra, of human frailties and vices. Even as the kohen gadol approaches the most sacred place on earth on the holiest day of the year, he cannot be assumed to act appropriately. The yetzer ha-ra lurks everywhere and confronts us at all times. No matter who we are or where we are, we need to struggle to overcome our negative tendencies and to follow the path which the Torah charts for us. This frightening realization, that there is no sanctuary to where we can flee from our own weaknesses, caused the elders to cry.