Zoroastrianism is named for its founder, Zoaoaster (Zarathustra), a prophet who lived in ancient Persia at some point between 1500 and 500 BCE. Like Ikhnaton in the 14th century BCE and  Muhammad in the 8th century CE, Zoroaster set out to transform the polytheistic folk traditions of his people into monotheism.  He taught that there was only one God,  Ahura Mazda, literally the “Wise Lord 

Zoroaster’s teachings are preserved in the Avesta, including the Gathas, a set of 5 hymns he himself is said to have composed.  The Gathas  and other scriptures were passed down thorugh the millennia by Zoroastrian priests.  In Yasna 30 of the Ahunavaiti Gatha we can read Zoroaster’s foundational message, that humans are called to choose between Good and Evil by the very structure of the universe: 

3. Now the two primal Spirits, who reveal themselves in vision as Twins, are the Better and the Bad, in thought and word and action. And between these two the wise ones chose aright, the foolish not so. 
4. And when these twain Spirits came together in the beginning, they created Life and Not-Life, and that at the last Worst Existence shall be to the followers of the Lie, but the Best Existence to him that follows Right. 
5. Of these twain Spirits he that followed the Lie chose doing the worst things; the holiest Spirit chose Right, he that clothes him with the massy heavens as a garment. So likewise they that are fain to please Ahura Mazda by dutiful actions.

Zoroaster addresses Ahura Maszda , “I recognize Thee, O Mazda, in my thought, that Thou the First art (also) the Last”  (Yasna 31, 8).  Ahura Mazda is the  Supreme God, glorious, omniscient, compassionate, wise.  He is pure light, life, and absolute goodness.  He cannot be the originator of evil.  But there is evil in the world, and according to Zoroastrian belief, evil was created by Ahura Mazda’s destructive twin,  Ahriman,  the lord of darkness, lies, evil, and death.  Ahura Mazda and Ahriman (Angra Mainyu ) are locked in an eternal  struggle, and human beings are called to join the forces of goodness to help Ahura Mazda overcome the powers of evil.  Zoroastrians believe that this cosmic war will, in fact, be won by Ahura Mazda and his supporters.  However, humans are envisioned as having free will.  They can serve truth or lie, goodness or evil, life or death in the everyday ethical choices they make.  After death their souls are judged on the basis of their actions, whether they did more good or evil in the world, and sent to heaven or hell accordingly. However, at the end of time humans, through their good thoughts and works will have helped destroy Ahriman and ultimately all souls will be redeemed, even those in hell.. 

For Zoroastrians, Ahura Mazda is symbolized by the living flames of a sacred fire. They perform ceremonies and say prayers before a fire. However, Zoroastrians no more worship fire than Christians worship the cross. Both symbols are used as visible signs of an invisible reality, a way of evoking the transcendent.