Zoroastrianism, the dominant pre-Islamic religious tradition of the Iranian peoples, was founded by the prophetic reformer Zoroaster in the 6th or 7th century BCE (if not earlier). The religion survived into the 20th century in isolated areas of Iran, and is also practiced in parts of India (particularly Bombay) by descendants of Iranian immigrants known as Parsis. For this reason, the religion as practiced in India is alternatively known as Parsiism.
Zoroaster (also known as Zarathushtra) was a priest who sought to reform aspects of the pre-Islamic pantheistic religion practiced in his community. Some of the practices he disapproved of included the sacrifice of animals (particularly bulls), as well as the ritualized consumption of the intoxicating beverage haoma. At the age of 30, Zoroaster experienced a vision in which the supremacy of the god of wisdom, Ahura Mazda, was revealed to him. The rest of the pantheon of deities was reduced to the status of demons and lesser spiritual creatures, with Angra Mainyu, or Ahriman, posited as the incarnation of evil standing in contrast to the goodness and light of Ahura Mazda. This dualism is often regarded as having been influential in the formulation of Jewish theology, and through Judaism, that of Christianity.
Zoroastrianism spread throughout Iranian lands, into Central Asia along trade routes, and further into East Asia. The Seleucids, Parthians, and Sassanians all practiced the faith. But as Richard C. Foltz has noted, Zoroaster's doctrine was not codified until sometime in the third century CE, under the Sassanians.1 Our historical understanding of the tradition is thus more accurately described as Sassanian Zoroastrianism, and we should assume that the religion had evolved, perhaps very significantly, in the millennium since the time of its founder.
Small temples dating to the pre-Islamic era have been found throughout Iran, and surviving records describe the installation of sculpture in Zoroastrian places of worship. None of these icons have survived, but since some ancient Iranian coins often include Greek-inspired imagery (particularly Parthian and Seleucid examples), it is conceivable that Zoroastrian temple sculpture of these periods may also have reflected a Hellenistic influence, perhaps similar to that found in Kushan-era Gandhara. The only Zoroastrian art still extant is found in coins, particularly those minted by Sassanian rulers. These coins regularly depict a fire altar flanked by two attendants, who may represent elite members of the Zoroastrian priesthood known as magi.
Historical commentary recorded by Hui-li and other Buddhist contemporaries of the seventh century often misinterpreted the religion (perhaps willfully) as focused upon the worship of fire. While fire is an important element in Zoroastrianism, it is not considered a deity in its own right. Rather, along with light, fire serves as an agent of purification and a symbol of the supreme deity. Three specific fires are named by Zoroastrian tradition and came to carry special cultic significance; these were the flames of Farnbag, Gushnasp, and Burzen-Mihr. The Farnbag fire was associated with the priesthood, and was first kept in Khwarezm. According to tradition it was transported a number of times since the sixth century BC, until it was moved to a permanent seat in the sanctuary of Kariyan in Fars (this location has not been identified). The Gushnasp fire was originally kept in Media as the fire of the magi, but in later centuries it became a symbol of monarchy. The fire altar on royal Sassanian coins included in this exhibit may depict the Gushnasp flame. The last fire, the Burzen-Mihr, was associated with the peasantry, and was ranked lower than the others. Localized "branch" fires of these main three were maintained in temples, royal palaces and in villages.2
It is possible that Zoroastrianism was carried by Iranian traders into East China as early as the sixth century BCE, and there may even be reason to believe that magi served in the court of the Western Zhou dynasty prior to the eighth century BCE.3 Some of the earliest firm evidence of Zoroastrian presence in China is found in the so-called "Ancient Letters," dated to around 313 CE and found near Lou-lan, demonstrate the presence of Sogdian Zoroastrianism in Xinjiang by the early fourth century.
References:
(1) Richard C. Foltz, Religions of the Silk Road: Overland Trade and Cultural Exchange from Antiquity to the Fifteenth Century (New York: St. Martin's Press, 1999), p. 28.
(2) Encyclopaedia Britannica Online.
(3) Victor Mair, "Old Sinitic *Myag, Old Persian Magus, and English 'Magician,'" Early China 15 (1990), pp. 27-47.