The book of Esther is not a historical document in the usual sense--but that doesn't undermine its importance as a religious book in the Jewish canon.
This article is excerpted from The JPS Bible Commentary: Esther, and is reprinted with permission of the Jewish Publication Society. Other excerpts from this article, focusing in greater detail on the comedic elements of the book, may be found in the "Holidays" section of MyJewishLearning.com, in connection with the holiday of Purim.
Why was Esther Written?
Megillat Esther, the Book of Esther in the form that we have it in the Hebrew Bible, provides the story of the origin of Purim, the blueprint for its celebration, and the authorization for its observance in perpetuity. The story itself is implausible as history and, as many scholars now agree, it is better viewed as imaginative storytelling, not unlike others that circulated in the Persian and Hellenistic periods among Jews of the Land of Israel and of the Diaspora.
This story seems to have been known in several different versions, or to have gone through a number of different stages in its development before it was linked with Purim and incorporated into the Bible. As a Diaspora story --a story about, and presumabIy, for, Jews in the Diaspora during the Persian period--it provides an optimistic picture of Jewish survival and success in a foreign land.
In this it resembles other Diaspora stories such as the biblical Book of Daniel (chapters 1‑6) and the apocryphal books of Judith and Tobit. But unlike those books, Esther lacks overtly pious characters and does not model a religious lifestyle. Esther is the most "secular" of the biblical books, making no reference to God's name, to the Temple, to prayer, or to distinctive Jewish practices such as kashrut.
Yet Esther, of all the biblical books outside of the Torah, is the only one that addresses the origin of a new festival. For this reason, if for no other, Esther should be considered a "religious" book. Its main concern, the very reason for its existence, is to establish Purim as a Jewish holiday, for all generations.
How Esther Establishes the Holiday of Purim
Megillat Esther establishes the Jewishness of the holiday by providing a "historical" event of Jewish deliverance to be commemorated and an authorization, through the letter of Mordecai, for the continued commemoration of the event. Just as the more ancient festivals are historicized and their observance is mandated by the Torah, so Purim is historicized and its observance is mandated by the Megillah. The Book of Esther serves as the authorizing document for Purim, a holiday that is not mentioned in the Torah.