Purim restores the merit of na'aseh v'nishma

בגמרא קימו וקבלו הדר קבלוה בימי אחשורוש כו'.

In the Gemara: "They fulfilled and accepted" — they re-accepted it (the Torah) in the days of Achashverosh, etc. (Shabbos 88a).

Chazal derive from "kimu v'kiblu" that in the time of the Purim miracle Bnei Yisrael re-accepted the Torah, now of their own free will.

ותוס' הקשו הא הקדימו נעשה לנשמע.

And Tosafos asked: but they already gave precedence to "we will do" before "we will hear" (na'aseh before nishma)!

Tosafos objects: at Sinai Bnei Yisrael willingly accepted the Torah with "na'aseh v'nishma," so what was lacking that needed re-acceptance at Purim?

אבל נראה שהקב"ה ראה שיחטאו אח"כ בעגל ויאבדו נעשה כמ"ש במדרש.

But it appears that the Holy One saw they would later sin with the Egel (Golden Calf) and would lose the "na'aseh," as it is said in the Midrash.

Hashem foresaw the cheit ha'egel, through which the merit of their wholehearted "na'aseh" would be forfeited.

אך על ידי שהקדימו נעשה במסירת נפשם לה'.

But because they gave precedence to "na'aseh" with mesirus nefesh (self-sacrifice) to Hashem.

Their original declaration of "na'aseh" was made with total self-sacrifice, and that devotion was not without consequence.

זכו שיכפה עליהם הר כגיגית.

They merited that the mountain was held over them like a barrel (Shabbos 88a).

As a reward for their self-sacrificing "na'aseh," Hashem also imposed the Torah upon them by force, "holding the mountain over them like a barrel."

פי' שניתן להם התורה בע"כ.

The meaning is that the Torah was given to them against their will (b'al korcham).

This coercion was itself a gift: the Torah was bound to them not only by their consent but by an unbreakable compulsion.

ולכך נשארה להם התורה גם אחר החטא.

And therefore the Torah remained with them even after the sin.

Because the Torah was also imposed by force — independent of their merit — it could not be lost even when the cheit ha'egel forfeited their voluntary acceptance.

שלא ניתן להם ע"י הקדמה זו רק בעל כרחם.

For it was not given to them through that precedence (of na'aseh) alone, but against their will.

The enduring tie to Torah rested on the coerced acceptance, which no sin could undo, rather than on the voluntary "na'aseh" alone.

ולכך נשאר להם כנ"ל.

And therefore it remained with them, as above.

Thus the forced acceptance is precisely what preserved the Torah for Bnei Yisrael through their failures.

אבל באמת כל חטא העגל נראה שהיה ע"י עמלק הרשע ימ"ש שלולי הוא שעירבב את בנ"י בדרך לא היה שום חטא.

But in truth, it appears that the entire sin of the Egel came about through the wicked Amalek, may his name be blotted out — for were it not for him, who confused Bnei Yisrael along the way, there would have been no sin at all.

The Sefas Emes traces the root of the cheit ha'egel to Amalek, whose interference and confusion of Bnei Yisrael was the true cause of their stumbling.

ולכך בכל עת שיש מחיית זרע עמלק נתקן חטא העגל.

And therefore, whenever there is a blotting-out of the seed of Amalek, the sin of the Egel is rectified.

Since Amalek caused the sin, every defeat of Amalek brings a corresponding tikkun for the cheit ha'egel.

וז"ש הדר קבלוה כו' שחזרו לזכות ההקדמה של נעשה לנשמע כנ"ל:

And this is what is meant by "they re-accepted it" — that they returned to the merit of having given precedence to "na'aseh" before "nishma," as above.

At Purim, with the downfall of Haman the Amalekite, the cheit ha'egel was rectified, and Bnei Yisrael regained the lost merit of their original, willing "na'aseh v'nishma" — now restored through love.

Summary: Tosafos asks why the Torah needed re-acceptance at Purim if Bnei Yisrael already chose it with "na'aseh v'nishma." The Sefas Emes answers that their self-sacrificing "na'aseh" earned the coerced acceptance ("the mountain like a barrel"), which alone kept the Torah with them even after the cheit ha'egel forfeited their voluntary merit. Since that sin was really caused by Amalek, the defeat of Haman the Amalekite at Purim rectified the Egel and restored the lost merit of their willing acceptance.