Kohanim Bearing the Ark
Requires Beit HaMikdash
BaMidbar 7:9 And to the sons of Kehat he did not give; since the sacred service was upon them, they carried on the shoulder.
We are commanded that the Kohanim are to bear the Ark upon their shoulders, when we wish to transport it from place to place. Although this Mitzvah was at the time laid upon the Leviim, this was only because of the limited number of Kohanim [then available], for Aharon was the first [Kohen Gadol].
In reality the fulfillment of this Mitzvah devolves upon the Kohanim, and it is they who are to bear [the Ark], as is clear from the book of Yehoshua (3:14) and the book of 2Shmuel (15:25). And when David commanded [the Kohanim and the Leviim] to bring up the Ark for the second time, the book of 1Divrei HaYamim records: "So the Kohanim and the Levi'im sanctified themselves to bring up the Ark of Hashem, the G-d of Yisrael.] And the children of the Leviim bore the Ark of G-d upon their shoulders with the bars thereon, as Moshe commanded according to the word of Hashem." (1Divrei HaYamim 15:14-15, 16) That is to say, the Kohanim sanctified themselves for the bearing of the Ark, while the Leviim did so for the purpose of accompanying the Kohanim and playing the music.