The mitzvah of reading the Megillah on Purim is not mentioned explicitly in the Torah. Nevertheless, the Jewish people undertook to do so, at that time and for all generations to come.

Although the reading of the Megillah was instituted by the Rabbinic Court of this world, the Talmud tells us that this enactment was endorsed by the Court of the Upper World. (Makot 23b, Shevuot 39a) The Talmud also tells us that G‑d indicated to Moses that the reading of the Megillah would be continued also in the future. (Megillah 19b)

The Megillah is not merely a chronicle of the events that transpired in Shushan at that time.

It is important to note that the Megillah is not merely a chronicle of the events that transpired in Shushan at that time. The Talmud tells us that the Megillah was written with divine inspiration: (Megillah 7a)

Rabbi Eliezer says: "The Book of Esther was written with ruach hakodesh (divine inspiration). For it is written, 'Haman thought to himself,' (Esther 6:6) a fact not discernable [to the author] other than through ruach hakodesh.

Rabbi Akiva says: "The Book of Esther was written with ruach hakodesh. For it is written, 'Esther found favor in the eyes of all who saw her', (Esther 2:15) a fact not discernible other than through ruach hakodesh.

Rabbi Yosi the son of Durmaskis says: The Book of Esther was written with ruach hakodesh. For it is written, 'In those days, while Mordechai was sitting in the king's gate, Bigsan and Seresh, two officers of the king, of the guards of the threshold became infuriated and conspired to harm the king, Achashverosh and the thing became known to Mordechai, and he told it to Esther, the queen, and Esther said it to the king in the name of Mordechai.' (Esther 2:21-22) How did the thing become known to Mordechai if not through ruach hakodesh?

Rabbi Meir says: The Book of Esther was written with ruach hakodesh. For it is written, 'But they did not stretch out their hands to the plunder.' (Esther 9:10) How did the writer of the Megillah know that even the Jews in distant places did not touch the plunder if not through ruach hakodesh?"

Rabbi Yehuda said in the name of Shmuel: "If I had been present at this conversation among Tanaim I would have told then that I have an even better proof. For it is written, 'They endorsed it, and the Jews took it upon themselves, and upon their offspring, and upon all that join with them, that none should fail to celebrate these two days', (Esther 9:27) and our rabbis have taken this to mean that the Court of the Upper World endorsed what the lower court took upon itself. How did the writers of the Megillah know this if not through ruach hakodesh?"

Rabah said: "I can refute all of these proofs except for Shmuel's proof."

…Rav Yosef said: "It can be proven from that which is written, 'These days of Purim will not pass from amidst the Jews'. (Esther 9:28) How could they know this if not through ruach hakodesh?"

Rav Nachman the son of Yitzchak said: "It can be proven from the conclusion of the verse, 'And their remembrance shall not cease from their offspring'". (Ibid)

 All festivals will be discontinued but for the days of Purim.

Our Sages have also inferred from this last verse that the Megillah would be read for all future generations. (Yerushalmi Megillah 1:5) Furthermore, the Midrash tells us that all festivals will be discontinued but for the days of Purim.1

All festivals will be discontinued but the days of Purim will never be discontinued, as it is written, 'And their remembrance shall not cease from their offspring'. (Esther 9:28)

…Rabi said: "Yom Kippur will also never be discontinued, as it is written, 'And this shall be for you for an everlasting law'." (Lev. 16:34)

It would appear to me that this passage is discussing the time the Jewish people will spend in exile. It is telling us that it is inevitable that their sinfulness will cause them to neglect the observance of the Festivals, as it is indeed written, "G‑d has made the Festivals and Shabbos be forgotten in Zion." (Lamentations 2:6) The Torah assures us, however, that the days of Purim will not suffer such neglect. It also assures us that Yom Kippur will atone for the Jewish people even if they do not do proper teshuvah.