The reason why we light the Menorah on Chanukah is to commemorate the great miracle that happened during the war against the Hellenists regarding the Temple oil. After the Greeks had defiled the Holy Temple during the battle with the Jews, only one flask of pure oil was found, enough the light the Temple menorah for only one day. Miraculously, the small amount of oil lasted for eight days.

The purpose of a miracle is to revive the faith of all that witness it. We must ask: why did G-d make this particular miracle happen at that time? Why was there a need to revive the faith of the Jewish people, and why did G-d choose the Temple oil as the means of strengthening their faith?

In Torah law, mere contact with an idolator is not enough to make oil impure—it would have to actually be offered to idols. It is the rabbis who said that the mere touch of an idolator makes the oil impure (See Talmud Nidah 34, Shabbos 17). Although they set up an idol of Zeus in the Temple, the Greeks did not take the containers of oil that they broke open, and make an offering from it. As such, the oil that they defiled was only considered to be impure by Rabbinic law. So had the rabbis not made their decree, the oil would have been perfectly acceptable—it would not have been impure at all!

At the time of the battle with the Greeks, the Jews had an internal dispute as well. A sect of Jews called the Sadducees was gaining in popularity. That sect accepted the authority of the Torah, of Biblical law, but rejected both the Oral Law given to Moses, as well as Rabbinic authority to interpret Biblical law and to enact decrees. So between the influence of the Greeks and that of the Sadducees, many began to question the validity and authority of the Oral Law and Rabbinic decrees.

This explains why G-d performed the miracle of the oil: it strengthened the faith of the Jewish People in the very area where it was weakest. As we explained, only Rabbinic decree made it necessary to obtain new oil—the oil they already had was acceptable according to the Torah. Per the Torah, no miracle was warranted. The miracle was only needed in order to follow the Rabbinic decree. Until new oil could be obtained that satisfied wholly Rabbinic concerns of impurity, the oil miraculously burned for eight days! (Based on the lectures of Rabbi Yaakov Yitzchok Ruderman ztl, quoting Sefer Pri Chadash and Pnei Yehoshua)

Thus the message of Chanukah is not only about the defeat of Hellenism and assimilation, but the validation of rabbinic authority to guide and direct the Jewish nation forward. They are the bearers of tradition, and it is their leadership which has ensured the survival of the Jewish people, thousands of years later.