What it was
Zoroastrianism is the religion founded on Zoroaster’s teaching. Its pillars are the worship of Ahura Mazda; the ethical triad of good thoughts, good words, good deeds; fire as the symbol of purity, tended in fire temples; and a cosmology of struggle between good and evil that ends in the world’s renewal. Its scripture, the Avesta, was transmitted orally for centuries before being written down, held to have happened under the Sasanians.
Role
Zoroastrianism was the dominant religion of the Achaemenid, Parthian and Sasanian empires, and under the Sasanians it functioned as a state religion with an organized priesthood — the magi, from whose Greek name the word magic derives. Its ideas of judgment after death, heaven and hell, a savior figure, and resurrection are often discussed as influences on Judaism, Christianity and Islam, though the degree of influence is debated.
Fate
After the Arab conquest, gradual conversion reduced Zoroastrians to a small minority; some emigrated to India from around the 8th–10th century, becoming the Parsis, who include prominent modern communities. Today small communities remain in Iran, notably in Yazd and Kerman, and in India, with a worldwide diaspora. Total numbers are small — commonly estimated at well under 200,000 — yet the faith survives as one of the world’s oldest continuously practiced religions.