(21) This song . . . shall not be forgotten out of the mouths of their seed.—And it is not forgotten now. St. Paul made special use of it in the last days of the second Temple. This song is a favourite piece of Hebrew poetry to this day. Rashi observes: “This is a promise to Israel that the law shall not be utterly forgotten by their seed.”

I know their imagination.—Heb., yêtzer, the same word employed in Genesis 6:5; Genesis 8:21. It is the word commonly used in Rabbinical literature for the evil nature or good nature in any man. The nature which they are forming, or making, this day, would be a literal rendering of the sentence in this verse. And yet with all this, He made Balaam say, “He hath not beheld iniquity in Jacob nor seen perverseness in Israel” (Numbers 23:21). Comp. 1Chronicles 28:9, “The Lord . . . understandeth all the imaginations of the thoughts,” and Psalm 103:14, “He knoweth our frame (yêtzer); He remembereth that we are dust.”

Deuteronomy 31:21. This song shall testify against them as a witness — That they were sufficiently admonished of their duty, and forewarned what would be the consequence of their defection from me and my worship, (Deuteronomy 32:18-19,) and be a clear evidence that the calamities which befall them are judgments sent from me for the punishment of their transgressions. How ought this to be remarked with wonder! For, to this very day, above three thousand years after, this song is a strong proof and demonstration, both to Jews and Christians, that Moses did indeed speak by the commandment of God. I know their imagination — Inclination to idolatry, which they do not check as they ought: and some of them do not only cherish it in their hearts, but, as far as they can and dare, secretly practise it, as may be gathered from Amos 5:25; Acts 7:43.

31:14-22 Moses and Joshua attended the Divine Majesty at the door of the tabernacle. Moses is told again that he must shortly die; even those who are most ready and willing to die, need to be often reminded of its coming. The Lord tells Moses, that, after his death, the covenant he had taken so much pains to make between Israel and their God, would certainly be broken. Israel would forsake Him; then God would forsake Israel. Justly does he cast those off who so unjustly cast him off. Moses is directed to deliver them a song, which should remain a standing testimony for God, as faithful to them in giving them warning, and against them, as persons false to themselves in not taking the warning. The word of God is a discerner of the thoughts and intents of men's hearts, and meets them by reproofs and correction. Ministers who preach the word, know not the imaginations of men; but God, whose word it is, knows perfectly.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 3:16.19. Now therefore write ye this song—National songs take deep hold of the memories and have a powerful influence in stirring the deepest feelings of a people. In accordance with this principle in human nature, a song was ordered to be composed by Moses, doubtless under divine inspiration, which was to be learnt by the Israelites themselves and to be taught to their children in every age, embodying the substance of the preceding addresses, and of a strain well suited to inspire the popular mind with a strong sense of God's favor to their nation. It shall not be forgotten: this seems not to be a precept that they should remember it, but a prediction, that God would give them sad occasion to remember it, by bringing upon them the dreadful calamities mentioned in it.

Their imagination which they go about, even now; either their inward inclinations to idolatry, which they do not check, as they ought, but rather entertain with delight; and some of them do not only cherish it in their hearts, but as far as they can and dare secretly practise it, as may be gathered from Amos 5:26 Acts 7:43; or their secret purposes to allow themselves therein, when they are settled in their land, which were clearly known to God, though it may be not fully evident to themselves.

And it shall come to pass, when many evils and troubles are befallen them,.... As did in the times of the judges, in the Babylonish captivity, and do now in their present exile:

that this song shall testify against them as a witness; which so clearly points at their sins, with all their aggravated circumstances, and describes so fully their calamities, distresses, and punishment for them:

for it shall not be forgotten out of the mouths of their seed; which shows that it respects time to come, their later posterity, whose memory of this song would be conjured up by the evils that should come upon them for their sins; nor is it forgotten by them to this day, who acknowledge there are some things in it now fulfilled or fulfilling in them:

for I know their imagination which they go about even now: or are "making" (c); forming and framing within themselves, there being a secret inclination in their minds to idolatry, which were working and contriving schemes to bring it about, and set it up; and this, God, the searcher of hearts, knew full well, and that in process of time this evil imagination would break forth into act, in an open and flagrant manner:

before I have brought thee into the land which I sware; to their fathers, to give it to them for an inheritance, as is suggested in Deuteronomy 31:20.

(c) "faciens", Montanus; "quam facit", Pagninus.

And it shall come to pass, when many evils and troubles are befallen them, that this song shall {l} testify against them as a witness; for it shall not be forgotten out of the mouths of their seed: for I know their imagination which they go about, even now, before I have brought them into the land which I sware.

(l) That these evils are come upon them, because they have forsaken me.