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.
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.