Zoroastrianism was founded in Persia in the 6th century BCE by the priest Zarathustra, known to the Greeks as Zoroaster. Zarathustra reformed existing Persian polytheism with his teachings about the highest god, Ahura Mazdā, and his primeval clash with Angra Mainyu, the Destructive Spirit. Zarathustra’s teachings grew to dominate Persia, but they fluctuated in popularity as various empires and cultures overtook the region.