We spend the entire month of Tishrei renewing and rejuvenating ourselves - spiritually and physically. After Rosh HaShana, Yom Kippur, Sukkot we end the month with Shmini Atzeret and Simchat Torah. I find it quite telling that the last thing we renew is the Torah. Our guide to life renews itself along with us.
In the last section of the Torah that we read we come across a strange phrase M'Yamino Aishdat Lamo(33:2). Specifically the word Aishdat is curious - Rashi explains that this phrase means that the Torah came from the fire. It is this immagery that Moshe is providing for us here - the Torah came from God through the fire. The Torah was forged in the fire of God providing us with a gift that is strong and lasting.
This is what Moshe is driving at by using the word Aishdat. Bnai Yisrael know by now that Moshe is leaving and Yehoshua is taking over. "I'm moving on; Yehoshua will be with you - yet the Torah that God gave to us will always be with you".
Rav Hirsch tellsu s that Dat is "the cosmic conception of the Torah" and Aish is "the power which gives movement...the dark invisible fire by which the eternal God-given laws of nature in all his creations". This is very similar to the Rambam's philosophy - that part of God's existence is the primordial mover, which provides the locomotion for the universe to function.
Moshe here is telling us that the best way we can understand the essence of God is through fire - not in the Zoroastrian sense - but through Aishdat. Moshe makes it very clear in combining these two words that we are not to separate these ideas, that they are intrinsically intertwined and that the best way to gain the true understanding of God is through the Torah.