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.
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.