(based on Zera Shimshon Parshas Emor pg. 59
Concerning the possuk “Speak to the Kohanim…” the Medrash comments, “The possuk (Tehillim 9/10) “The fear of Hashem is pure, it stands forever” is referring to this possuk (“Speak to the Kohanim…”). Rav Levi explained, “Through the fear that Aharon had of Hashem he merited that this parsha will be pertinent for him, his children, and his subsequent generation until the end of time… like it says “Speak to the Kohanim (the sons of Aharon) …” (Medrash Rabbah (26/6))
Zera Shimshon asks why does the possuk specifically mention that the Kohanim are the sons of Aharon? Isn’t this common knowledge that the children of Aharon are Kohanim?
Secondly, how does the possuk “The fear of Hashem is pure, it stands forever” explain the possuk “Speak to the Kohanim…”. They seem to be totally unrelated?
He answers that Aharon and subsequently all the Kohanim are like malachim with an intrinsic kedusha (holiness). Therefore one might think that if they defile that kedusha even one time (when they come in contact with a corpse) then the defilement would be terminal; they would remain tommai their whole lifetime.
It would also then follow that nowadays when there is no way for a Kohain to purify himself there would be no prohibition for a Kohain to become tommai since everyone is a tommai maiss today.
This however is not the case. We know that even today kohanim are commanded not to become tommai.
Why is this? Zera Shimshon explains that the participants in the incident of the Golden Calf are responsible that there is a prohibition for a Kohain to become tommai maiss (the tummah one gets by being in contact with a corpse)! Hashem abolished the Mallach HaMovess (Angel of Death) at the time of the Giving of the Torah. No Mallach HaMovess, no tummas mais.
However, after B’nai Yisroel worshipped the Golden Calf, Hashem brought back the Mallach HaMovess and with it tummas maiss. Therefore they who took part in the Goldern Calf are responsible that tummas maiss exists.
Aharon also took part in the Golden Calf however he feared Hashem and was embarrassed for what he did (Refer to Vayikra 9/7 Rashi and Ramban). The Gemorra tells us that Hashem forgives someone who is embarrassed of a sin that he did.
The Gemorra also tells us that one who does teshuva because of the love that he has for Hashem (and not only because he fears Hashem’s punishment) is so powerful that not only does Hashem forgive him for what he did , but that sin turns into a merit for him!
Aharon’s fear of Hashem was not only because of Hashem’s punishment but rather he feared Hashem’s greatness (yiras haromamus) and this , explains Zera Shimshon, is just like teshuva for the love of Hashem.
In light of the above, concludes Zera Shimshon, we can now understand the Medrash. The Medrash was bothered by why the Torah added the extra phrase “the sons of Aharon“ after calling them Kohanim. The answer is that it is to stress that this prohibition will be in force until the end of time.
But the Medrash is stilled bothered by why should this be? Why isn’t their kedusha defiled after being tommai one time and therefore it would not be applicable today.
The Medrash answers is that the embarrassment of Aharon stemmed from yiras Hashem of Aharon and was pure (yiras haromamus) and therefore the whole incident of the Golden Calf, which brought tummah into the world, was not only not held against him but is considered a merit for him!
Therefore tummas maiss cannot defile his intrinsic kedusha. It goes even farther. His teshuva (being embarrassed of what he did because of his fear of Hashem) was so powerful that it also weakened the power of tummas maiss to defile his future generations.
This then is the meaning of the possuk, “The fear of Hashem is pure (the fear of Aharon) it stands forever (even for his future generations)!