Devarim 31:21

21 And it shall come to pass, when ra’ot rabbot and tzarot are befallen them, that this shirah shall testify against them as an ed (witness); for it shall not be forgotten out of the mouths of their zera; for I know their yetzer (inclination) in which they go about, even now, before I have brought them into ha’aretz which I swore.

Devarim 31:21 Meaning and Commentary

Deuteronomy 31:21

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" ^F3; 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 ) .

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FOOTNOTES:

F3 (hve) "faciens", Montanus; "quam facit", Pagninus.

Devarim 31:21 In-Context

19 Now therefore write ye this shirah for you, and teach it the Bnei Yisroel; put it in their mouths, in order that this shirah may be an ed (witness) for Me against the Bnei Yisroel.

20 For when I shall have brought them into ha’adamah which I swore unto Avotav (their fathers), zavat cholov u’devash (that floweth with milk and honey); and they shall have eaten and filled themselves, and grown fat; then will they turn unto elohim acherim, and serve them, and provoke Me, and break My Brit.

21 And it shall come to pass, when ra’ot rabbot and tzarot are befallen them, that this shirah shall testify against them as an ed (witness); for it shall not be forgotten out of the mouths of their zera; for I know their yetzer (inclination) in which they go about, even now, before I have brought them into ha’aretz which I swore.

22 Moshe therefore wrote this shirah on that day, and taught it the Bnei Yisroel.

23 And he gave Yehoshua ben Nun a charge, and said, Chazak! And of a good courage; for thou shalt bring the Bnei Yisroel into Ha’Aretz which I swore unto them; and I will be with thee. [Isa 49:8]