The first time Moshe has a divine revelation he is at the burning bush, astonished to discover that the bush is burning with fire, “but the bush is not consumed”. At the end of his life, however, In the book of Deuteronomy, Moshe refers to G-d as a "consuming fire", This begs the question: is G-d a consuming fire or a fire that does not consume?
As the Bible tells it G-d is a consuming fire. Any revelation of G-d causes the soul to try to escape the confines of the physical and become one with the fire that “consumes” the confining material reality. This is what the Jewish people experienced at the revelation at Sinai when the people cried to Moshe that man cannot see G-d and survive. This is what happened to the children of Aron, who died because of the ecstasy of their spiritual connection to G-d. And this is exactly what G-d does not want.
G-d's first message to Moshe was that the “bush is not consumed” - the fire should not lead you to escape the physical. The consuming fire of G-d must burn in your heart, yet, paradoxically, you cannot be consumed. You may be the greatest prophet of all times, you may be the lawgiver, you may speak to G-d “like a man speaks to his friend”, but you may not be consumed by the fire. You must not abandon the reality in which you live, you must not forget about the people around you, you must be like the flame surging upward yet grounded by it's wick.
Only at the end of his life when his soul is about to depart from this world and unite the infinite light of G-d does he articulate the the truth as he is about to experience it. Only when he knows that his mission on this earth is complete, only when he is preparing to return his soul to it's father in heaven does he speak of G-d's true essence, saying that the G-d is indeed a consuming fire.