Why does the Megila say "a Jewish man was in the capital of Shushan and his name was Mordechai," when there were many Jewish people who lived in Shushan? The sages explain that many Jew fled Shushan out of fear of Haman; Mordechai was one of the few who did not. Rav Yonasan Eybeshitz disagrees with this interpretation, because later on in the Megila, Esther instructs Mordechai to "go and assemble all the Jews," which demonstrates that there were many Jewish people to be found around Shushan.
Therefore, Rav Eybeshitz explains that the Jewish people in Shushan were "hidden Jews," they did not "wear their Yiddishkeit on their sleeves." Many attempted to conceal their Judaism by integrating into society. Mordechai, however, understood the danger of such attempts. He understood that for the sake of preservation, one must be proud of what the Torah has to offer. This is why the Megila says that Mordechai "was in the capital of Shushan," to be understood literally. He did not remain insular, rather he entered the marketplaces and the social clubs as a religious Jew, demonstrating to his ashamed brethren the value of being subservient to God. Mordechai showed his people that only their allegiance to God would ensure their preservation.
The Megila refers to Mordechai as "ish Yehudi – man of Judah" and "ish Yemini – man of Benjamin." How could Mordechai have been from both the tribe of Judah and the tribe of Benjamin? Rav Eybeshitz explains based on the Midrash which says that one can change the "Yehudi" into the word "yihudi", from the word "ehad – unique and one."
Mordechai was from Benjamin, but the Megila describes him as "yihud – one" because he was the one Jew who in turn publicized the "oneness", "uniqueness", and sovereignty of God.
Mordechai would publicly wear his tzitzit and tefillin, the signs which demonstrated that there is only one God. As the Jewish people attempted to hide from who they were by eating and drinking from utensils of the Holy Temple, indulging in Ahasuerus's lavish feast, Mordechai loomed over their consciousness as he donned his tallit and tefillin. His steadfast conscription to the symbols of Halacha and Torah Judaism saved and sustained the Jewish people then, and continues to guarantee our eternal existence, despite the threats of the modern day Amalek.