"The best life is for the one,
who turns towards light,
and sheds light to others." Zarathustra
Fire as a Symbol of Everything Sacred
In ancient times, when Zoroastrians built no temples, possessed no religious imagery and had no books on the teachings of the faith, light served as the focus of their religious practices. Fire (athra / atarsh / atash) was a means of producing light.
When using a flame, a source of light, as the focus while contemplating the spiritual aspects of one's life, the symbolisms carried by the fire & the light it produced, conveyed some of the essential principles of the faith. For instance, carrying a fire into a dark place dispels the darkness giving us the metaphor of the light of wisdom banishing the darkness of ignorance.
From wisdom are derived the principles of justice and order. The temporal fire was also the symbol of the cosmic fire of creation, a fire that continues to pervade every element of creation. In this sense, fire takes on a much broader meaning than a flame. Light and fire were also essential elements for sustaining life.
Reference: Zoroastrian Heritage