There is a popular Medrash which is found in Tractate Shabbat (83a).
The Torah says, “And Moses brought forth the people out of the camp to meet G-d; and they stood at the lowermost part of the mount” (Exodus 19:17). Rabbi Avdimi bar Ḥama bar Ḥasa said: the Jewish people actually stood beneath the mountain, and the verse teaches that the Holy One, Blessed be He, overturned the mountain above the Jews like a barrel, and said to them: If you accept the Torah, excellent, and if not, here will be your burial.
bar Ya’akov said: From here we may deduce that this can be used as
“a Declaration of Protest” to the obligation to fulfill the Torah. The Jewish people can claim that they were coerced into accepting the Torah, and it is therefore not binding.
Rava said: Even so, they again accepted it willingly in the time of Ahasuerus, as it is written: “The Jews ordained, and took upon them, and upon their seed, and upon all such as joined themselves unto them.” (Esther 9:27)
Over the centuries, Rabbis and scholars have tried to grapple with the true meaning of this text. After all, the more popular understanding of our receiving of the Torah was that we recited “Na’aseh V’nishma” or ‘we fully accept the Torah, sight unseen!’ In other words, we weren’t coerced at all!
Perhaps we may suggest that the term Torah here may refer to a specific “Torah” or “doctrine” which we truly accepted wholeheartedly at Sinai, but without much understanding of its ramifications. In the wake of the Exodus and the great many open miracles, the people were intoxicated with the notion of G-d and his Torah, and they couldn’t imagine that a time may come in which they would not be able to sustain it. It never occurred to them that they might even briefly ever abandon the Torah, or their fidelity to the Ribono shel Olam.
But over the millennia, the people realized that the actual fulfillment of the Torah is not as easy as it looks, and indeed, after the destruction of the first Temple, they convinced themselves that G-d had totally abandoned them. They forgot (or chose to forget) their Sinaitic covenant which states that G-d would never ever abandon the Jewish people under any circumstances. In the days of Achashveirosh, the time seemed ripe for total assimilation and the total abandonment of any notion of G-d’s eternal fidelity, either for His people or for their destiny.
But after having witnessed the miracle of Purim, they understood that even during the bitter exile when they tangibly felt the loss of the Divine presence in their hearts, G-d had never left them and He never would! His presence may not have been as apparent and miraculous as in the days of the Exodus, but nevertheless He was there behind the scenes pulling the strings. And He always would be there for them, so long as they just opened their eyes. The people wholeheartedly reaccepted the Torah-the Torah of G-d’s great love for his people!