Deuteronomy 31:19

Write ye this song - The song which follows in the next chapter. Things which were of great importance and of common concern were, among the ancients, put into verse, as this was found the best method of keeping them in remembrance, especially in those times when writing was little practiced. Even prose was sometimes sung. The history of Herodotus was divided into Nine books, and each inscribed with the name of one of the Nine Muses, because these books were anciently sung. Homer is reported to have sung his poems through different Greek cities. Aristotle observes that anciently the people sung their laws. And Cicero observes that it was a custom among the ancient Romans to sing the praises of their heroes at the public festivals. This was the case among the northern inhabitants of Europe, particularly in Ireland and Scotland; hence the Gaelic poetry of Ossian and others. See Dodd; and see the note on Exodus 15:1, where the subject is largely treated.

A witness for me against them - i. e., an attestation from their own mouths at once of God's benefits, their own duties, and their deserts when they should fall away. Being in verse it would be the 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.

31:19 Write this song - Which is contained Deuteronomy 32:1 - 43, and is put into a song that it may be better learned, and more fixed in their minds and memories. Put it in their mouths - Cause them to learn it, and sing it one to another, to oblige them to more circumspection.A witness - Of my kindness in giving them so many blessings, of my patience in bearing so long with them, of my clemency in giving them such fair and plain warnings, and my justice in punishing such an incorrigible people.