Zoroastrians affirm a cosmic struggle between good and evil. The supreme God, Ahura Mazda (the Wise Lord), is wholly good and the source of light, truth, and life; opposed to him stands the destructive spirit, Angra Mainyu (Ahriman), the origin of the lie, darkness, and death. The world is the arena of their conflict, in which every person, by their choices, takes a side. The tradition teaches that the struggle is not eternal in its outcome: good is destined to triumph, evil to be overcome, and creation to be renewed at the end (the frashokereti).
The doctrine's central text is Yasna 30.3-4 in the Gathas, where the two primal spirits are described choosing, from the beginning, between life and not-life, good and evil, a passage scholars of Zoroastrianism, including Mary Boyce and R. C. Zaehner, have debated at length over how strictly dualistic to read it. Some interpreters treat Ahura Mazda and Angra Mainyu as two independent, co-original powers, a stronger dualism; others, especially later Zoroastrian theology, read Angra Mainyu as ultimately a lesser, dependent force whose defeat is assured because Ahura Mazda alone is truly uncreated and supreme, a reading that keeps the tradition closer to a qualified monotheism, and the two interpretations continue to divide scholarly and traditional accounts of the religion's basic theological structure.