Deuteronomy 31:9

And Moses wrote this law, and delivered it unto the priests the sons of Levi, which bare the ark of the covenant of the LORD, and unto all the elders of Israel.

Moses wrote this law, and delivered it unto the priests. The law thus committed to writing was either the book of Deuteronomy only, as Kurtz and Delitzsch think, or the important part of it contained between Deuteronomy 27:1 and Deuteronomy 30:1 (as Davidson maintains; Horne's 'Introduction,' 3: pp. 616, 618), or the whole Pentateuch, as the phrase, "this law," 'the book of this law,' signifies in the later historical books (Joshua 1:8; Joshua 8:31; Joshua 8:34; Joshua 24:26; 2 Kings 14:6: see further in the General Introduction).

It was usual in cases of public or private contract for two copies of the engagement to be made-one to be deposited in the national archives, or some secure place for reference, should occasion require; the other to remain in the hands of the contracting parties (Jeremiah 32:12). The course was followed on this renewal of the covenant between God and Israel. Two written copies of the law were prepared, the one of which was delivered to the public representatives of Israel-namely, the priests and the elders.

The priests ... which bare the ark of the covenant. In all ordinary journeys it was the common duty of the Levites to carry the ark and its furniture (Numbers 4:15), but on solemn or extraordinary occasions that office was discharged by the priests (Joshua 3:3; Joshua 6:6; 1 Chronicles 15:11). Hence, frequent allusion is made to the priests as invested with the high function of keeping the canon of inspiration (Deuteronomy 17:18).

All the elders of Israel. They were assistants to the priests, and overseers to take care of the preservation, rehearsal, and observance of the law.