Deuteronomy 33:8

In blessing the tribe of Levi, Moses expresses himself more at large, not so much because it was his own tribe (for he takes no notice of his relation to it) as because it was God's tribe. The blessing of Levi has reference. I. To the high priest, here called God's holy one (Deu 33:8), because his office was holy, in token of which, Holiness to the Lord was written upon his forehead. 1. He seems to acknowledge that God might justly have displaced Aaron and his seed, for his sin at Meribah, (Exo 17:7), which might be very remarkable, and which God might have an eye to in conferring the priesthood upon him, though no mention is made of it there. All the Chaldee paraphrasts agree that it was a trial in which he was found perfect and faithful, and stood in the trial; therefore not that, Num 20:2. He prays that the office of the high priest might ever remain: Let thy thummim and thy urim be with him. It was given him for some eminent piece of service, as appears, Mal 2:5. "Lord, let it never be taken from him." Notwithstanding this blessing, the urim and thummim were lost in the captivity, and never restored under the second temple.

— Matthew Henry's Concise Commentary (public domain)

of Levi he said--The burden of this blessing is the appointment of the Levites to the dignified and sacred office of the priesthood (Lev 10:11; Deu 22:8; Deu 17:8-11), a reward for their zeal in supporting the cause of God, and their unsparing severity in chastising even their nearest and dearest relatives who had participated in the idolatry of the molten calf (Exo 32:25-28; compare Mal 2:4-6).

— Jamieson-Fausset-Brown Commentary (public domain)