Torah Light Testifying Through Concealment
בפסוק וענתה השירה הזאת לפניו לעד כי ל"ת מפי זרעו כו'
Regarding the verse, "And this song shall testify before them as a witness, for it shall not be forgotten from the mouth of their offspring" (Devarim 31:21).
The Sfas Emes opens with the verse that promises the Torah's song will remain forever as a witness among Bnei Yisrael, never forgotten by their descendants.
דכ' אנכי הסתר אסתיר פרשנו שיהי' נסתר ההסתר ויאמרו כי אין אלקי בקרבי כו'
It is written, "And I will surely hide My face" (Devarim 31:18). We have explained that the very hiddenness itself becomes hidden, so that they will say, "It is because Hashem is not in my midst" (Devarim 31:17).
Hashem warns that He will hide His face, and the Sfas Emes explains this means the hiddenness becomes doubly concealed, to the point that a person no longer even senses that Hashem is hidden and mistakenly concludes that Hashem is absent.
ושמעתי בשם הרה"ק מפרשיסחא ז"ל שזה הוא נחשב לחטא כי צריכין להאמין כי השי"ת עמנו בצרה
And I heard in the name of the holy Rebbe of Peshischa, of blessed memory, that this itself is reckoned as a sin, for one must believe that Hashem is with us in our distress, as it says, "I am with him in trouble" (Tehillim 91:15).
The Rebbe of Peshischa taught that this very conclusion is itself a sin, because a Yid is obligated to believe that Hashem accompanies him even in the midst of his troubles.
וע"י התורה ושירה הזאת יכולין לברר זאת כי השי"ת עמנו
And by means of the Torah and this song, one is able to clarify this point: that Hashem is with us.
Through the Torah and the song of Haazinu, a person is able to clarify and recognize the truth that Hashem remains with us even in concealment.
וז"ש וענתה השירה כו' לעד שע"י התורה יכולין לברר הכל כי תורה אור
And this is the meaning of "And this song shall testify... as a witness" (Devarim 31:21) — that through the Torah one is able to clarify everything, for the Torah is light, as it says, "For a mitzvah is a candle and the Torah is light" (Mishlei 6:23).
The verse calls the song a "witness" because the Torah, being light, illuminates and clarifies everything, dispelling the false impression that Hashem has abandoned us.
ואין החושך יכול להחשיך אור התורה כמ"ש עבירה מכבה מצוה ואינו מכבה תורה
And the darkness cannot darken the light of the Torah, as Chazal said: an aveirah extinguishes a mitzvah but does not extinguish Torah (Sotah 21a).
An aveirah can extinguish the light of a mitzvah, but it has no power to extinguish the light of Torah, so the darkness of galus can never overcome Torah's illumination.
וזה עצמו כי ל"ת מפי זרעו זה העדות שהשי"ת עמנו כמ"ש חז"ל על משכן העדות עדות היא שהשכינה שורה בישראל
And this itself is the meaning of "for it shall not be forgotten from the mouth of their offspring" (Devarim 31:21) — this is the testimony that Hashem is with us, as Chazal said regarding the Mishkan of Testimony: it is a testimony that the Shechinah rests among Bnei Yisrael (see Shemos 38:21 with Rashi).
The promise that the song will never be forgotten from the mouths of our offspring is itself the standing testimony that the Shechinah dwells among Bnei Yisrael, just as the Mishkan was called the Mishkan of Testimony.
וזה בלוחות וכמו כן בכל התורה בכללות בנ"י
And this was so in the Luchos, and likewise it is so in the Torah as a whole, with respect to the entirety of Bnei Yisrael.
This testimony was present in the Luchos, and it carries through the entire Torah as it relates to the collective body of Bnei Yisrael.
וכמו כן בפרט לפי התגלות הארת התורה בלב איש ישראל עדות הוא למעט דביקות אשר לו כנ"ל:
And likewise it is so in particular, according to the revelation of the illumination of the Torah within the heart of each individual Yid — it is a testimony to the measure of dveikus that he possesses, as explained above.
On the individual level, the degree to which the Torah's light shines in a particular Yid's heart is itself a testimony to the measure of dveikus that he has attained with Hashem.
Summary: The Sfas Emes addresses the deepest danger of galus, when Hashem's hiddenness becomes so concealed that a person fails to sense it at all and falsely concludes that Hashem has abandoned him. Citing the Rebbe of Peshischa, he teaches that this conclusion is itself a sin, for a Yid must believe that Hashem is with us even in our distress. The remedy is the Torah and the song of Haazinu, which serve as an eternal witness, for the Torah is light that no darkness and no aveirah can ever extinguish. This enduring presence of Torah among Bnei Yisrael is the very testimony that the Shechinah dwells with them, as in the Mishkan of Testimony. And on the personal level, the measure of Torah's illumination glowing in each Yid's heart bears witness to the dveikus he has attained.