At that time — i.e., at Sinai, after Moses’ second descent from the mount, not at the time of Aaron’s death. Yet the death of Aaron and the separation of the tribe of Levi are similar events in their way: both alike lose territorial inheritance through bearing the burden of the Law.
To bear the ark of the covenant of the Lord, to stand before the Lord to minister unto him, and to bless in his name. — A recent critic has said that the writer of Deuteronomy knows no distinction between priests and Levites. (See on this point Deuteronomy 11:6.) Rashi’s note on this verse is better: “To bear the ark (He separated) — the Levites; to stand before Jehovah to minister to Him, and to bless in His name — the priests.”