(Adapted from an article by Rabbi M. Miller in his Sefer - 'Yom Tov Shiurim.')
Purim is a holiday of great excitement and exhilaration. Let us attempt, however, to look below the surface and see how these seemingly superficial sensations can be utilized in our spiritual striving. The pasuk states (Shemos 19:17) that at the time of the giving of the Torah Israel was positioned beneath Har Sinai. Rashi on this verse comments that either Bnei Yisrael were standing at the foot of the mountain, or a Midrash understanding, that Hashem gave Bnei Yisrael a choice to either accept the Torah and live, or if not, there would the be buried and returned to a state of Tohu V'Vohu.
The Gemara in Shabbos 88a furthers this point. Rav Avidmi says "this comes to teach us that God held the mountain over them like a barrel, saying 'If you accept the Torah, well and good, but if not there will be your burial place. Rav Acha the son of Yaakov says "Nevertheless they reaccepted it during the days of Achashverosh as it is written, 'the Jews flilfilled and accepted - they fulfilled that which they had previously accepted."
The verse in the Megillah states Kiymu V'Kiblu "they fulfilled and accepted." This wording seems to be out of order since normally acceptance must precede fulfillment. Here, however, the natural order is reversed. This verse must therefore be understood in the light of a previous acceptance of the Torah at Har Sinai, the fulfillment of which now took place as a validating conclusion to the process.
An obvious question in the aforementioned piece of Gemara springs to mind. If the first valid act of acceptance of the Torah is recognized as having occurred only in the days of Achashverosh, how could Bnai Yisrael have been deemed culpable for transgressions committed prior to that era? In the times of the prophets, for example, the people were frequently admonished and punished. How are these two concepts of the invalidity of the acceptance of the Torah at Har Sinai and liability for transgressing the Torah to be reconciled?
The following simple parable may serve to elucidate this point. If a person is threatened at knife point and forced to eat some desirable item of food, the fact that he consumes it voraciously does not demonstrate his craving for this particular food. If however at a later date he purchases this item of food of his own volition, that would confirm not only the fact that he presently enjoys this food, but more importantly, that he actually liked the food when his life was under threat. This practical illustration clarifies for us the way in which re-acceptance of the Torah during the time of Achashverosh operated as a retroactive proof of the peoples' actual willingness to accept it at Har Sinai when, to all appearances, they were coerced into doing so.