The vast majority of commentators assume that Esther was a religiously observant Jew. They claim, that, though not explicitly mentioned in the text, she managed to keep Torah laws in the palace, and the hiding of her identity emanated not from shame but from an attempt to maintain her observance. The Megillah tells the story of how a righteous heroine saves her people from disaster.
A minority opinion suggests, in contrast, that Esther had assimilated into Persian society as evidenced by her Persian name, Esther = Ishtar. The absence of religion in the scroll might reflect her own lack of observance. The events of the story teach that assimilation does not protect one from antisemitism and tell a tale of Esther's reconnecting to her Jewish roots.
Esther kept Mitzvot – While many of these commentators1 say explicitly that Esther kept all the Mitzvot, the others mention only specific commandments that she observed such as Shabbat, kashrut, family purity, and eliminating leavened bread before Pesach.2
Esther was forced to not observe them – RashiMegillah 13a s.v. כתליAbout R. Shelomo Yitzchaki3 claims that Esther was compelled not to keep Mitzvot in the palace. He apparently assumes, though, that when she was in Mordechai's house she kept everything.
How did Esther keep Mitzvot:
Shabbat
Hid the fact – Rava in Bavli Megillah13a says that Esther was able to keep Shabbat because she had seven maidservants, one for each day of the week. One explanation of his words is that the seventh woman was not suspicious of Esther's behavior on Shabbat, being unaware that it was any different from the way she acted all week long.4
Others kept her secret – Esther might have revealed her observance to Hegai or her servants, but they kept it to themselves. According to some, they did so because Hegai himself was Jewish and the seven maidens had been converted by Esther.5
Kashrut – Rav and R. Yochanan in the Bavli Megillah13a13b14a assert that Hegai gave Esther kosher food. R. Avigdor Kohen Zedek explains that Hegai believed that non-kosher food was a cause of brazenness and therefore refrained from serving it.6 Alternatively, as above, Hegai and the maids might have been Jewish and kept Esther's secret.
Marrying Achashverosh – See Esther's Relations with Achashverosh for the various justifications of Esther's marrying a non-Jew. According to some commentators,7 the ends justified the means, while others8 propose that Esther's actions were under duress.
Esther's name – Ibn Ezra suggests that Esther is the Persian translation of the Hebrew name Hadassah. It is the predominant name in the Megillah because this was the name by which she was known in the palace. It is likely that she chose to be so called as part of her attempts to hide her Jewish identity.9
Concealing identity – Ibn Ezra suggests Mordechai's commandment was meant to ease Esther's keeping of commandments. Had Achashverosh known her Jewish identity, he would have watched carefully and forced her not to observe. See Why Conceal Esther's Nationality for elaboration and additional opinions.
Religious identity of the whole nation – Esther Rabbah 7:12 and 7:13 present two contradictory portraits of the nation:
According to 7:12, Esther was representative of a similarly observant nation. When Haman complained to Achashverosh about the Jews being different, he was referring to their keeping of Shabbat and holidays.
Esther Rabbah 7:13, in contrast, suggests that the Jews were hedonistic like their Persian neighbors, joining in Achashverosh's party and abandoning their religion.
Absence of Hashem in the Megillah – The absence of Hashem's name does not connote a lack of religiosity, but might rather be an attempt to prevent desecration of Hashem's name.10
Biblical parallels – This position views Esther as similar to Daniel. He too was placed in a position of power in exile, and is explicitly described as observing Hashem's commandments even at the potential cost of death.
Keeping Mitzvot – According to this approach, Esther was not a particularly observant Jew. R. Reggio points out that there is no evidence in the Megillah of her attempting to keep any of the commandments, and by hiding her identity she only made it harder to do so.13 In addition, the Megillah does not mention that Mordechai warned Esther before he separated from her not to betray her faith, as one would have expected of a religious Jew.
Marrying Achashverosh – Esther might have had no problem marrying Achashverosh and even desired the glory of being chosen as queen.14
Esther's name – The name Esther is the name of a Persian god.15 It is possible that Esther changed her original name, Hadassah, to the more Persian name either because she was ashamed of her Judaism, or simply wanted to better fit into Persian society. In contrast to Yosef (Bereshit 41:45) and Daniel (Daniel 1:7), who are given their second names by Paroh or the minister, Esther seems to already have had her second name,16 suggesting that she was assimilated even before being chosen as queen.
Concealing identity – R. Reggio says Mordechai thought that revealing her religion (which was disdained by the Persians) would hinder her chances of becoming queen. Alternatively, R. Medan suggests Esther was embarrassed by her Jewish identity and wanted to conceal it.
Religious identity of the whole nation – R. Medan points out that the people of Persia at the time of the story were people who had the opportunity to return to Israel with the proclamation of Cyrus, but decided against doing so. This, perhaps, suggests that they were not the most committed of Jews.
Absence of Hashem in the Megillah – This position might suggest that the absence of Hashem's name in the Megillah reflects the absence of Hashem in Esther and the nation's lives.
Biblical parallels – One might suggest that Esther is very similar to Yosef.17 Both were important figures in a foreign palace who might have originally attempted to deny their Jewish roots,18 but eventually had to face their true identities and return to their nations.
Evaluation of Esther – R. Medan points out that Esther should be evaluated based not on her early deeds but her final actions. Though she might have been assimilated, at the crucial moment she returned to her Jewish roots and helped save her nation.
Purpose of the Megillah – One of the lessons of the Megillah might be that assimilation does not prevent antisemitism; it does not distinguish between observant and less observant Jews.