When Moshe was sending the 12,000 man army to war against the Midianites, he instructs Pinchas to take the "holy vessels/articles" and the trumpets in his hand. What were the holy vessels? Could this be the ark of the covenant? Any insights?
Rashi's explanation is
וכלי הקדש AND THE VESSELS OF HOLINESS … [WERE WITH THEM] — these were the Ark and the Golden Plate (which are termed קדש, “holiness”, in Numbers 4:20 and Leviticus 8:9 respectively) (cf. Sifrei Bamidbar 157:4; Sotah 43a).
The Ibn Ezra agrees that it was the Ark.
The Haamek Davar says it was the Ark or the Tzitz.
Rabbeinu Bahya agrees with that, saying
וכלי הקודש, “and the holy vessels, etc.” This is a reference to the Holy Ark and the golden headband of the High Priest (Sotah 43). Tanchuma Mattot 5 relates that the Midianites employed all kinds of charms in conjunction with Bileam and that the kings were riding in the air when they saw this war. Moses had told the soldiers that if they noticed that the kings of the Midianites would employ such means against them they should show the ציץ, and the Midianites would immediately fall to the ground.
The Shadal explains that it couldn't have been the Ark, the Tzitz or the Chatzotzrot, so he doesn't know and suggests it might have been the Urim VTumim.
The talmud, in Sotah 43a says it was the Ark and the stone tablets within.