Deuteronomy 31:19

"And now," sc., because what was announced in Deu 31:16-18 would take place, "write you this song." "This" refers to the song which follows in ch. 32. Moses and Joshua were to write the song, because they were both of them to strive to prevent the apostasy of the people; and Moses, as the author, was to teach it to the children of Israel, to make them learn it, that it might be a witness for the Lord (for Me) against the children of Israel. "This" is defined still further in Deu 31:20, Deu 31:21 : if Israel, through growing satisfied and fat in its land, which was so rich in costly good, should turn to other gods, and the Lord should visit it in consequence with grievous evils and troubles, the song was to answer before Israel as a witness; i.e., not only serve the Lord as a witness to the people that He had foretold all the evil consequences of apostasy, and had given Israel proper warning (Knobel), but to serve, as we may see from Deu 31:20, Deu 31:21, and from the contents of the song, as a witness, on the one hand, that the Lord had conferred upon the people so many benefits and bestowed upon them such abundant blessings of His grace, that apostasy from Him was the basest ingratitude, for which they would justly be punished; and, on the other hand, that the Lord had not rejected His people in spite of the punishments inflicted upon them, but would once more have compassion upon them and requite their foes, and thus would sanctify and glorify Himself as the only true God by His judgments upon Israel and the nations. 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…

In this chapter Moses, having finished his sermon, I. Encourages both the people who were now to enter Canaan (Deu 31:1-6), and Joshua who was to lead them (Deu 31:7, Deu 31:8, Deu 31:23). And, II. He takes care for the keeping of these things always in their remembrance after his decease, 1. By the book of the law which was, (1.) Written. (2.) Delivered into the custody of the priests (Deu 31:9, and Deu 31:24-27). (3.) Ordered to be publicly read every seventh year (Deu 31:10-13). 2. By a song which God orders Moses to prepare for their instruction and admonition. (1.) He calls Moses and Joshua to the door of the tabernacle (Deu 31:14, Deu 31:15). (2.) He foretels the apostasy of Israel in process of time, and the judgments they would thereby bring upon themselves (Deu 31:16-18). (3.) He prescribes the following song to be a witness against them (Deu 31:19-21). (4.) Moses wrote it (Deu 31:22). And delivered it to Israel, with an intimation of the design of it, as he had received it from the Lord (Deu 31:28, etc.).

31:19-22 When sung by the collective community in days to come, this song (32:1-43) would remind the people of God’s faithfulness in the past and of their too-frequent disloyalty. The major blessings for obedience and curses for disobedience were implicit in the song. The people of Israel would sing it as a sign that they had placed themselves under these promises and threats. Through music, they would be reminded of God’s blessings for obedience and would invoke God’s judgment should they prove unfaithful.