On Yom Kippur, the High Priest was allowed to enter The Holy of Holies, the most sanctified part of the temple. In that room, the Ark of the Covenant was kept. When the high Priest entered, he was required to remove all of his golden garments. He could only answer with the white linen clothes of a basic Priest. The Talmud gives the reason: the prosecution cannot become the defense. What does this mean?
The Tabernacle and its requirements are written before the Golden calf episode in the Bible. Nonetheless, the Tabernacle was actually commanded after the Golden calf episode, not before. The reasons for this are beyond the scope of this article. There are many elements of the Tabernacle and its services that are intended to atone for that tragic sin.
Thus, the very material of gold reminds one of the Golden calf. It is a "prosecutor", in that it brings up that painful episode again. Yom Kippur is the day when the Golden calf episode was forgiven, and when Moses returned from Mount Sinai with the second set of the stones of the 10 Commandments. Thus, at the heart of that day's service, is forgiveness for the sin of the calf. The high priest enters the holy of holies, where those stones of the Commandments are housed in the Holy Ark. It would be inappropriate for the high priests to wear gold at that moment. The prosecutor, i.e. the gold, cannot become the defense in this atonement ceremony.
But what about Aaron? He himself would seem to be part of the prosecution, as it was he who made the calf! Now it is true that many commentaries mitigate Aaron's role in the calf, and explain how he was attempting to dissuade the people, to delay them until Moses could return, and so forth. I'm sure that Aaron was not an idolater and very much made this calf against his will, but the Bible is clear that he made it intentionally and did not put up a fight. So why is he allowed to enter the Holy of Holies on Yom Kippur?
The priests come from the family of Levi, the tribe of people who did not participate in the worship of the Golden calf. Originally, it was to be the first born sons who were to perform the Temple service. Since, however, they did participate in the Golden calf, this task was taken away from them and given to the Levites. So we see that service in the Tabernacle was contingent upon not having participated in the sin of the Golden calf.
So what about Aaron!? Why is he not excluded? Was this candidate not properly vetted?
Of course he was, and this is the unique power of the Jewish view on life. Judaism does not look at a person's past, it looks at a person's character and their potential for the future. And in Judaism, love conquers all.
Aaron was the paradigm of love. Our sages teach us to "Be among the disciples of Aaron: love peace, pursue peace, love people and bring them close to Tora." The job of the priests in the Tabernacle required love of Israel, love of humanity. This Aaron had in abundance, more than anyone else. In fact, one could say that it was Aaron's love of the people that led him to make the calf. He did not initiate it, it certainly grieved him deeply. When explaining his deed to Moses, he said, "You know this people, that they are in a bad way."
Good and bad are concepts that express more than simply a value judgment. During the days of creation, the day when things were separated, Monday, is never described as being "good." That is because separation, while at times necessary, is not good, any more than getting a cavity filled, while necessary, is pleasureful. God is one, he desires his children to be one. When there is war, strife and fratricide, it is not good, it is bad. Aaron was telling Moses that the people were splitting, were turning against each other. It was clearly the responsibility of those who desired "A God who will go before us, for we do not know what has happened to this Moses."
Aaron did whatever he could to prevent this greatest evil, civil war. The real dividers were the sinners, including the first born, who thus disqualified themselves from working in the Tabernacle. Aaron was a unifier, and if the price to salvage the People's unity was making a calf, he was prepared to do it. For Aaron, love conquered all.
Twice daily the Jew proclaims his faith. He does so by reciting the famous Shema prayer, "Hear, oh Israel, the Lord our God, the Lord is one." This proclamation is immediately followed by a command: "and you shall love thy Lord with all of your hearts, with all of your soul, and with all of your strength." The first thing a Jew needs to put in his heart is love. Love conquers all.
That is not to say that idolatry, licentiousness, and other sins are given a free pass. It is to say that God has lots of patience for those things, but not for hatred and violence towards our fellow human beings. Love and peace are at the top of the list, and for the High Priests, they are the most basic job requirements.