Ahura Mazda, the wise lord, is uncreated, wholly good, and the source of everything good. Ranged against him is Angra Mainyu, the destructive spirit, who is not his creation, not his equal, and destined to lose. This is what makes the tradition hard to classify. It is monotheist in that only one being is worshipped and only one is the origin of creation, and dualist in that evil is a genuinely independent principle rather than a shadow of the good or a test permitted by it. Zoroastrians generally describe their faith as monotheist; scholars have argued the point for two centuries without settling it.
Around Ahura Mazda stand the Amesha Spentas, six beneficent immortals who are at once aspects of him and guardians of parts of creation: good purpose, best order, desirable dominion, holy devotion, wholeness and immortality, attached respectively to cattle, fire, metals, earth, water and plants. The scheme means that treating the physical world well is not incidental to the religion but is the religion, since the elements are the bodies of divine beings.
Human beings matter to the outcome. Creation is a battlefield the hostile spirit invaded, and each person, through good thoughts, good words and good deeds, the formula that recurs throughout the liturgy, either strengthens or weakens the order Ahura Mazda established. Asceticism has no standing here. Marriage, children, agriculture and honest work are religious acts, and deliberate self-denial is closer to a fault than a virtue.
The end is not a rescue of souls from a doomed world but the repair of the world itself. Frashokereti, the making wonderful, is the final renovation in which evil is destroyed, the dead are raised in their bodies, and creation is restored to the perfection it was made with. A saviour figure, the Saoshyant, presides over it. What survives is this world, purified, not an alternative to it.
A late current known as Zurvanism made Ahura Mazda and Angra Mainyu twin offspring of Zurvan, infinite time, which resolved the dualism by subordinating both. It is attested largely in foreign and hostile reports, and it left no living successor.