In the ancient world, it was often believed that all material things consisted of the combinations of four elements--earth, fire, air and water
In primitive societies all of these elements were held to be sacred; in more "advanced" societies earth and air seemed to lose some of their aura of sanctity -- earth, perhaps, because it seemed so solid and common; air, perhaps, because it was so rarified and easy to take for granted. Fire and water retained their significance, real and symbolic in nearly all religious traditions.
For Rumi, burning represented the intense emotion of the lover and fire the annihilation of the self as it moves toward unity with the one
Fire also represents for many believers the tortures of hell. No wonder ordinary believers often fear and distrust the mystics--they seem to seek the very condition they should be trying to avoid. When Rabia or Catherine of Genoa implies that they would gladly burn in hell if that should be God;s will, they seem to be denying the very purpose of monotheistic religion, at least in its Islamic or Christian forms.
Moses' burning bush was not consumed, or even scorched so far as i know. Blake is only one of many visionaries who experienced tongues of fire--good or bad, i am not told. Blake seems not to distinguish or care, which makes him like most mystics. Fire just is!
Like many people, i fear uncontrolled fire. Every summer is fire season in the Western U.S. Every year homes and lives are destroyed. When i was ten, the building next to our home, a roller skating rink which had been converted to an arsenal for the (WWII) duration, burned to the ground., there was not only fire but exploding ammunition. I was old enough to be scared. We had to go stay with the neighbors at three am and i had my first cup of coffee, which in retrospect seems as memorable as the fire. It is hell of a poet who can spiritualize fire for me. Rumi is a hell of a poet!