Zoroastrian theology is strongly dualistic. In his visions, Zarathustra was taken up to heaven, where Ahura Mazda revealed that he had an opponent, Aura Mainyu, the spirit and promoter of evil. Ahura Mazda charged Zarathustra with the task of inviting all human beings to choose between him (good) and Aura Mainyu (evil). Consequently, Zoroastrianism is a highly ethical religion. Zarathustra taught that humans are free to choose between right and wrong, truth and lie, and light and dark, and that their acts, words, and thoughts would affect their lives after death. He was thus the first to promote a belief in two heavenly judgments: of the individual soul right after death and of all humankind after a general resurrection. His ideas of heaven, hell, and the resurrection of the body profoundly influenced Judaism, Christianity, and Islam. Later Zoroastrianism conceived of an opposition between body and soul, though there was no suggestion in its theology that the body was evil and the soul was good. A wandering preacher from Mesopotamia named Mani developed those theories into an extreme form of dualism called Manichaeism.
The Zoroastrian "Avesta" ("Book of the Law") is a fragmentary collection of sacred writings divided into: liturgical works with hymns ascribed to Zarathustra; invocations and rituals to be used at festivals; hymns of praise; and spells against demons and prescriptions for purification. Compiled over many centuries, the Avesta was not completed until Persia's Sassanid dynasty (226-641 C.E.).
Zoroastrianism all but disappeared in Persia after the Muslim invasion of 637 C.E. Only about 10,000 survive in remote villages in Iran, but over the centuries many sought religious freedom in India.