When God instructs the Children of Israel to make the menorah, He tells them to gather oil “for kindling lamps regularly” (Exodus 27:20). Rashi explains that the light was to be lit all night, every night, though not necessarily during the day. The lights of the menorah needed to be extinguished, cleaned, and refilled with oil every morning or afternoon. In this regard, the light of the menorah was similar to the Pillar of Fire which accompanied the congregation of Israel through the desert for 40 years, appearing at sundown and leaving with the dawn. Of course, the lamp was not for God’s use. But since the menorah stood inside the Tabernacle in a section only accessed by the Kohanim (priests) during the day, neither was the light intended for any practical human purpose.
The Hebrew of this phrase, l’haalot ner tamid, also means “for kindling an eternal flame.” Based on this verse we learn that there was a light on the menorah that was lit continually. According to the Talmud, this eternal light was the westernmost branch of the menorah. It was filled with the same amount of oil as the other branches, but while they were predictably extinguished by morning, it remained lit until the following evening and was the source of fire for the lighting of the other branches. While this branch was also refueled at the same rate as the other branches it remained continuously lit, and thus had a miraculous, Divine element to it.
The eternal light hinted to the Burning Bush, the form in which God first revealed Himself to Moses. This image was repeated for all of Israel at Mount Sinai, when the entire mountain was aflame but not consumed. This image was again repeated in the miracle of Hanukkah in which one flask of pure oil lasted a supernatural eight days.
This image speaks deeply to the spiritual nature of man, as described in Proverbs:
The lifebreath of man is the lamp (ner) of Hashem Revealing all his inmost parts. Proverbs 20:27
Hassidic masters teach that Man is like a flame. The wick represents his ephemeral physicality attaching him to this world, while his eternal soul, expressed as a flame, strives upward. It is Man’s earthly side that requires sustenance, i.e. oil. But this oil must be pure in order to serve God, and therefore a servant of Hashem is limited to eating Biblically pure food. But both physicality and spirituality, wick and flame, are necessary, and necessarily interconnected in order to bring God’s light into the world. Neither can exist without the other. Mitzvoth (Torah commandments) are composed of the spiritual intention acting upon the physical world. Hassidic tradition also explains that the unintentional habit to sway while praying represents this flickering between two worlds, vacillating between heaven and earth.
The eternal flame is still central to Judaism. Every synagogue has an eternal light which hangs over the ark containing the Torah scrolls, a tradition going back at least six hundred years.