Yoma, Chapter One, Mishnah Five
As I stated in my commentary to mishnah two and in my introduction to the tractate, many of the high priests were Sadducees, or perhaps more accurately, had Sadducean leanings. The Sadducees debated with the Pharisees the proper form of the Yom Kippur ritual, as I explain below. In the mishnahs eyes, the elders spend a week teaching the high priest how to perform the service in the proper Pharisaic manner. Our mishnah describes the parting words between the elders and the high priest, where they remind him to act like a Pharisee and not a Sadducee.
1) The elders of the court handed him over to the elders of the priesthood and they took him up to the upper chamber of the house of Avtinas.
2) They adjured him and then left. And they said to him [when leaving]: Sir, high priest, we are messengers of the court and you are our messenger and the messenger of the court. We adjure you by the one that caused His name dwell in this house that you do not change anything of what we said to you.
3) He turned aside and wept and they turned aside and wept.
Section one: On the eve of Yom Kippur the elders would give him over to the elders of the priesthood. He would go up to the house of Avtinas where the incense was normally made. There he would learn how to make the incense that he was going to make on Yom Kippur within the Holy of Holies.
Section two: When the elders of the court depart from him they recite to him the following oath, reminding him that he must do whatever they have taught him to do. The Talmud explains that there was the following debate between the Sadducees and the Pharisees. The Sadducees held that the high priest would put the incense into the incense pan outside of the Holy of Holies and then go inside, whereas the Pharisees held that this was done inside.
Section three: In the climactic conclusion to the mishnah, both the high priest and the elders end up crying (this would make a good movie). He cries because they suspected him of being a Sadducee and they cry because reality forced them to suspect him. Alternatively, they cry because the situation in their time had deteriorated so badly that they had to force a high priest to take an oath that he would not act like a Sadducee.