The law, with its commandments, promises, and threats, was already a witness of this kind against Israel (cf. Deu 31:26); but just as in every other instance the appearance of a plurality of unanimous witnesses raises the matter into an indisputable truth, so the Lord would set up another witness against the Israelites besides the law, in the form of this song, which was adapted to give all the louder warning, “because the song would not be forgotten out of the mouths of their seed” (Deu 31:21). The song, when once it had passed into the mouths of the people, would not very readily vanish from their memory, but would be transmitted from generation to generation, and be heard from the mouths of their descendants, as a perpetual warning voice, as it would be used by Israel for God knew the invention of the people, i.e., the thoughts and purposes of their heart, which they cherished (עשׂה used to denote the doing of the heart, as in Isa 32:6) even then before He had brought them into Canaan. (On Deu 31:20, vid., Deu 7:5; Deu 9:5, and Exo 3:8.) - In Deu 31:22 the result is anticipated, and the command of God is followed immediately by an account of its completion by Moses (just as in Exo 12:50; Lev 16:34, etc.). - After this command with reference to the song, the Lord appointed Joshua to the office which he had been commanded to take, urging him at the same time to be courageous, and promising him His help in the conquest of Canaan. That the subject to ויצו is not Moses, but Jehovah, is evident partly from the words themselves, “I will be with thee' (vid., Exo 3:12). (Note: Knobel's assertion (on Num 27:23) that the appointment of Joshua on the part of Moses by the imposition of hands, as described in that passage, is at variance with this verse, scarcely needs any refutation. Or is it really the case, that the installation of Joshua on the part of God is irreconcilable with his ordination by Moses?)
verses 24-27
With the installation of Joshua on the part of God, the official life of Moses was brought to a close. Having returned from the tabernacle, he finished the writing out of the laws, and then gave the book of the law to the Levites, with a command to put it by the side of the ark of the covenant, that it might be there for a witness against the people, as He knew its rebellion and stiffneckedness (Deu 31:24-27). על־ספר כּתב, to write upon a book, equivalent to write down, commit to writing. תּמּם עד, till their being finished, i.e., complete. By the “Levites who bare the ark of the covenant” we are not to understand ordinary Levites, but the
