שמות יט, יז וַּיוֹצֵא משֶה אֶת -ה ע ם לִקְרַּאת ה אֱלֹקים מִן-הַּמַּחֲנֶה וַּיִתְיַּצְבוּ בְתַּחְתִית ה ה ר
Chaza”l tell us that the mountain was lifted over our heads at Har Sinai in order to compel us to receive the Torah. In a certain sense we thus received the Torah in a state of אנוס (forced). Later, the Jewish nation did accept the Torah willingly in the days of Esther (9, 27) קייימו וקבלו היהודים, (as explained by Rashi) that from the love of that miracle, the Jews now accepted the Torah willingly.
Perhaps the derivation from this verse in Esther can be understood. The usual manner is for some to receive something and then fulfill it. However, the order is reversed (first קיימו and then קבלו, similar to נעשה ונשמע). Thus, we can understand the verse in Megilas Esther to read we established first (willingly) that which we already received (with force).
With this understanding, it is possible to explain a Gemara (Nedarim 8a). Rav Gidel in the name of Rav learns the concept of permission to take an oath to keep a Mitzvah from the verse said by Dovid (תהילים קי"ט ק"ו) נשבעתי ואקיימה לשמור משפטי צדקך. And the Gemara asks on Rav Gidel, were not the Jewish people already sworn when they stood at Har Sinai (to keep the Mitzvos), and one oath cannot go into force when an oath already exists. And the Gemara answers that one is allowed to make an oath on the same Mitzvah, in order to provide additional catalyst to do the Mitzvah.
The question asked of Rav Gidel, could also have been asked directly about Dovid. However, according to what was explained earlier in the times of Dovid, the Jews still had “only” accepted Mitzvos by the “force of the mountain over their heads”, since Dovid lived about 500 years prior to the miracle of Purim. Thus, in the time of Dovid the people were able to take an oath to reinforce doing a Mitzvah.
Why did HaShem present the Torah to the Jews at Har Sinai in this manner of “force”? The concept of receiving Torah needed to be done in a manner not subject to the intellect of the people. Thus, the acceptance of Torah applied equally to all.
