And it shall come to pass, when many evils and troubles are befallen them, that this song shall 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. This song shall testify against them - Because in it their general defection is predicted, but in such a way as to show them how to avoid the evil; and if they did not avoid the evil, and the threatened punishment should come upon them, then the song should testify against them, by showing that they had been sufficiently warned, and might have lived to God, and so escaped those disasters.
Related Commentaries of Deuteronomy 31:21
Deuteronomy 31:21
21 And it shall come to pass, when many evils and troubles are befallen them, that this song shall testify againstc 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.