Albert Barnes 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)

Your holy one - that is, Levi, regarded as the representative of the entire priestly and Levitical stock that descended from him. The contrast between the tone of this passage and that of Genesis 49:5-7 is remarkable.

Although Jacob's prediction regarding the dispersion of this tribe proved true, it was overruled in such a way that it resulted in honor and reward. The recovery of God’s favor can be attributed to the faithfulness with which Moses and Aaron, who were from this tribe, served God in their high offices. It can also be attributed to the zeal and constancy that prominent members of the tribe (for example, Phinehas, Numbers 25:11 and following) and the whole tribe itself demonstrated on critical occasions in supporting the leaders of the people.

These same reasons led to Levi being selected for the special service of God in the sanctuary (Deuteronomy 10:8 and following, and Numbers 8:5 and following) and for the office of instructing their brothers in the knowledge of the Law. The events at Massah and Meribah—one occurring at the beginning and the other toward the end of the forty years of wandering—represent the entire series of trials by which God tested and exercised the faith and obedience of this chosen tribe.