Albert Barnes Commentary Deuteronomy 31:14-23

"And Jehovah said unto Moses, Behold, thy days approach that thou must die: call Joshua, and present yourselves in the tent of meeting, that I may give him a charge. And Moses and Joshua went, and presented themselves in the tent of meeting. And Jehovah appeared in the Tent in a pillar of cloud: and the pillar of cloud stood over the door of the Tent. And Jehovah said unto Moses, Behold, thou shalt sleep with thy fathers; and this people will rise up, and play the harlot after the strange gods of the land, whither they go to be among them, and will forsake me, and break my covenant which I have made with them. Then my anger shall be kindled against them in that day, and I will forsake them, and I will hide my face from them, and they shall be devoured, and many evils and troubles shall come upon them; so that they will say in that day, Are not these evils come upon us because our God is not among us? And I will surely hide my face in that day for all the evil which they shall have wrought, in that they are turned unto other gods. Now therefore write ye this song for you, and teach thou it the children of Israel: put it in their mouths, that this song may be a witness for me against the children of Israel. For when I shall have brought them into the land which I sware unto their fathers, flowing with milk and honey, and they shall have eaten and filled themselves, and waxed fat; then will they turn unto other gods, and serve them, and despise me, and break my covenant. And it shall come to pass, when many evils and troubles are come upon them, that this song shall testify before them as a witness; for it shall not be forgotten out of the mouths of their seed: for I know their imagination which they frame this day, before I have brought them into the land which I sware. So Moses wrote this song the same day, and taught it the children of Israel. And he gave Joshua the son of Nun a charge, and said, Be strong and of good courage; for thou shalt bring the children of Israel into the land which I sware unto them: and I will be with thee." — Deuteronomy 31:14-23 (ASV)

The transaction recorded in these verses may be regarded as the solemn inauguration of Joshua to the office to which he had some time before been called (Numbers 27:22), and his recognition in it by God. These were manifested by his being summoned into the tabernacle with Moses while the Lord appeared in the pillar of cloud (Numbers 12:5).

The future apostasy of the people is announced in Joshua's presence so that he might be fully aware of the danger and strive in his day to avert it. This he faithfully did ; but we find him in his own last address to Israel repeating the very same prediction and warning (Joshua 23:15–16).

A witness for me against them—that is, an attestation from their own mouths of God’s benefits, their own duties, and their deserts when they should fall away. Being in verse, it would be more easily learned and kept in memory. The use of songs for such didactic purposes was not unknown to the legislators of antiquity. Compare also the advice of Paul, teaching and admonishing one another in psalms and hymns and spiritual songs (Colossians 3:16).

He gave—that is, the Lord gave.