Ahura Mazda is fire. The Jews worship their God as a pillar of fire. The Christians declare that their God is a consuming fire. Fire symbolised the brilliance of the Lord. It is purifying.
Fire stands for effulgence, illumination.
The worship of Agni or Fire comes in the Vedas also. In every religion fire finds a place in rituals. Fire brings the message that God is Light of lights. Zoroaster himself held Fire in great sanctity as a noble gift from God. He summoned Fire from the sky and pointed it to heaven.