Charles Ellicott Commentary Deuteronomy 33:8

"And of Levi he said, Thy Thummim and thy Urim are with thy godly one, Whom thou didst prove at Massah, With whom thou didst strive at the waters of Meribah;" — Deuteronomy 33:8 (ASV)

And of Levi.—Next to Joseph, this tribe has the largest share in Moses’ last words, as might naturally be expected, since it was his own tribe. The character of the priest is the principal subject. The blessing may be paraphrased this way: “Let your Thummim and your Urim (the chief high-priestly ornaments) be always with some saintly man of yours, like him whom you (Israel) tested at Massah, and with whom you contended at the waters of Meribah (Moses’ own departed brother Aaron is alluded to, as the people murmured against them both in both places), like him (Eleazar or Phinehas) who said to his father and to his mother, ‘I have not seen him,’ and so on.

These are the priests who will teach Jacob your judgments and Israel your law.” The conduct of the tribe of Levi at Sinai is alluded to, when they stood by Moses and killed the idolaters. We are not told who headed them on that occasion. Eleazar or Phinehas may be intended. The conduct of Phinehas (in Numbers 25) is also a case in point. As Rashi observes, “his father and his mother, his brothers and children” cannot be taken literally, because the tribe of Levi on the whole was faithful. The fathers, mothers, brothers, and children chiefly belonged to the other tribes.

Let your Thummim and your Urim.—See Exodus 28:30. “Your Thummim and your Urim” may refer to Israel, to Levi, or to Jehovah Himself. In the last case, He must be thought to have tried Levi at Massah, and contended with Moses and Aaron at the waters of Meribah. It is not at all easy to distribute the pronouns with certainty in this speech.

If the writer of Deuteronomy was unaware of any difference between priest and Levite, how is the mention of Urim and Thummim to be explained?