Zarathushtra reformed the fire cult and made it the symbol par excellence of Ahura Mazda. In his Gathas he speaks of fire as a bright and powerful creation of Ahura Mazda and prefers fire instead of idols as a symbol of divinity. In its simplest form, fire is that which burns and gives out light. Fire can consume all organic matter and is able to transform most inorganic matter. The burning may be likened to that which destroys evil by consuming or changing. The light may been seen as that which makes life possible by providing heat and energy that permeates all of Ahura Mazda’s creations. Light in its various manifestations, whether it be from the fiery substance in the depths of the earth, or the glow of the sun, or the twinkle of the stars, is emblematic of Nature and Ahura Mazda. In his Gathas, Zarathushtra mentions Fire eight times in Yasna 31.3, 31.19, 34.4, 43.4, 43.9, 46.7, 47.6 and 51.9. Fire is described as bright, warm and energetic faculty of Ahura Mazda. Zarathushtra refers to the Divine Fire as the agency that derives its power form Asha (Truth, Immutable law) and rewards the righteous and the truthful. The epithet for fire in the Gathas is athro asha aojanho(the true strong fire).

Yasna 34.4, a verse in the Atash Nyaesh, says:

at tôi âtrêm ahurâ aojônghvañtem ashâ usêmahî asîshtîm êmavañtem stôi rapañtê cithrâ-avanghem at mazdâ daibishyañtê zastâishtâish dereshtâ-aênanghem. (in Avestan)

Thus, O Ahura we yearn through Asha for thy Fire, mighty, most-enduring and courageous, giving clear guidance in life to the earnest believers; but O Mazda to those with destructive tendencies, it overcomes their violence by the power of its flames. (English translation from "The philosophical, spiritual and ethical interpretation of the Gathas of holy Zarathushtra" by Framroz Rustomjee).

In the above verse, Zarathushtra tells us that Athra (Fire) gives clear guidance and help to the faithful by illuminating the path of Asha which is easily perceived by the truthful. The help that Athra gives is reserved for the person whose actions are in accordance with Asha for the help itself is in accordance with Asha and cannot violate the natural law, nor can it protect those who are lazy, irrational or evil. Thus, Athra is the faculty of Ahura Mazda that guides, illuminates and protects those who use their Vohu Mano (Good Thinking) to understand Asha and can only help those who work to promote Asha. Zarathushtra made fire the symbol of his religion, a symbol which in terms of sublimity, grandeur and purity is unequal by any of its kind in this world. He did not enjoin the worship of fire but only of Ahura Mazda whose very nature is eternal light.

The zest for fire in Zoroastrianism became the quest for truth. In the Atash Nyaesh, fire is figuratively mentioned as tava atarsh puthra Ahurae Mazda (thou fire son of Ahura Mazda). This is similar to the methaphors 'Father of Truth' (Yasna 44.4 and 47.3) and 'Father of Good Thought' (Yasna 31.8 and 45.4) that Zarathushtra uses in his Gathas to refer to Ahura Mazda. The Avesta tells us that just as fire can burn and destroy physical impurity in the same way it can remove spiritual uncleanliness or sin. The fire of Ahura Mazda is mighty through Asha and will bring manifest joy unto the righteous but a visible harm unto the wicked (Yasna 43.4). At Frasho Kereti it is through fire that profit will be bestowed upon righteous and the wicked will be destroyed (Yasna 51.9). According to the creation story in Bundahishm, when Ahura Mazda fashioned the seventh creation, Fire, it permeated into the other six creations, made them dynamic and brought them to life. The Amesha Spenta, Asha Vahista, who represents Best Truth is the protector of fire and is assisted by the Yazatas Adur, Srosh and Behram.

A Zoroastrian is not a fire worshiper but through the veneration of fire is able to generate an intimate communion with Ahura Mazda. When Zoroastrians stand in devotion before a sacred fire they believe that they are standing in the presence of the radiating power of Ahura Mazda. When they pray before a fire they pay homage to the creation that represents life and the inherent nature of Ahura Mazda -- total goodness.

In his immortal epic, the Shahnameh, Firdosi repels the charge of fire-worship often hurled against the Zoroastrians by his words: Na gui ke atash-parasta budand Parastanda a pak yazda budand, which means, do not say they (Zoroastrians) are fire worshipers, for they worship only God, the Holy.