Zoroastrianism was founded by the prophet Zarathustra — Zoroaster in Greek — somewhere in ancient Iran or Central Asia, probably between 1500 and 600 BCE. At its height under the Achaemenid and Sassanid empires it was the state religion of the largest empire the ancient world had seen. Today perhaps 100,000–200,000 Zoroastrians survive, concentrated in India (the Parsis) and Iran. What makes this religion remarkable is not its size but its influence: more ideas now central to Western religion entered through Zoroastrianism than through almost any other channel.
Zarathustra — a priest in the ancient Indo-Iranian religion — received a vision in which he was brought before Ahura Mazda, the Wise Lord, the supreme and wholly good deity. Ahura Mazda revealed to him the nature of reality: that the cosmos is the arena of a cosmic struggle between Asha (truth, order, righteousness) and Druj (lie, chaos, deceit). Every human being, in every thought, word, and deed, participates in this struggle.
Opposed to Ahura Mazda is Angra Mainyu (the Destructive Spirit — later called Ahriman) — not an equal opposite, but a lesser force of destruction that chose negation. The dualism of Zoroastrianism is not absolute: Ahura Mazda will ultimately prevail. But in the present age the struggle is real and human choice genuinely matters to its outcome.
This I ask, Ahura — tell me truly: who is the father of Asha, at the beginning? Who established the course of the sun and stars? Who is it through whom the moon waxes and wanes? Who holds the earth below and the firmament from falling? Who sustains the waters and the plants?
— Zarathustra, Gathas (Yasna 44)The Jewish exile in Babylon (586–538 BCE) occurred under Neo-Babylonian rule but ended under Cyrus the Great of Persia — a Zoroastrian who liberated the Jews and funded the rebuilding of the Temple. During and after this period, Jewish thought absorbed several ideas with no clear earlier precedent in the Hebrew scriptures: a cosmic adversary figure (Satan, from the Hebrew for "adversary," who in earlier texts is a member of God's court, not an opponent), a hierarchy of angels, bodily resurrection, final judgment, and apocalyptic eschatology — all of which are prominent features of Zoroastrianism.
These ideas passed from Jewish into Christian and Islamic thought, making Zoroastrianism a silent architect of much of what Western civilisation considers foundational religious belief. The Magi who visit the infant Jesus in Matthew's gospel are almost certainly Zoroastrian priests from the east — a detail that, taken seriously, makes the Nativity story a Zoroastrian prophecy about the Jewish messiah.