Zoroastrian tradition holds that Zarathustra, known in Greek as Zoroaster, received his revelation from Ahura Mazda, the Wise Lord, at the age of thirty, while drawing water from a river for a spring purification ritual. In the encounter, Zarathustra is said to have been led into the presence of Ahura Mazda and several attendant divine beings, the Amesha Spentas, who revealed to him the nature of the cosmic struggle between truth, asha, and falsehood, druj, and commissioned him to teach humanity how to align itself with the former.
The clearest and earliest evidence for Zarathustra's teaching survives in the Gathas, seventeen hymns composed in an archaic form of Avestan and later incorporated into the Yasna, the central liturgical text of the Avesta. The Gathas present a demanding ethical monotheism centered on Ahura Mazda as the sole uncreated creator god, opposed not by an equal rival power but by the destructive principle of falsehood that human beings choose to align with or against through their own thought, word and deed.
Zarathustra's teaching met resistance in his own lifetime according to tradition, and his early following remained small until, in the fullest form of the story, a local ruler named Vishtaspa accepted the new faith and extended it royal protection, allowing it to establish itself and eventually spread across the Iranian world.