The Zoroastrian concept of God incorporates elements of both monotheism and dualism. 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).
Though Zoroastrianism was never as aggressively monotheistic as Judaism or Islam, it does represent an original attempt at unifying under the worship of one supreme god a polytheistic religion comparable to those of the ancient Greeks, Latins, Indians, and other early peoples.
Its other salient feature, namely dualism, was never understood in an absolute, rigorous fashion. Good and Evil fight an unequal battle in which the former is assured of triumph. God's omnipotence is thus only temporarily limited.
Zoroaster taught that man must enlist in this cosmic struggle because of his capacity of free choice. Thus Zoroastrianism is a highly ethical religion in which the choice of good over evil has cosmic importance. Zarathustra taught that humans are free to choose between right and wrong, truth and lie, and light and dark, and that their choices would affect their eternal destiny.
The Zoroastrian afterlife is determined by the balance of the good and evil deeds, words, and thoughts of the whole life. For those whose good deeds outweigh the bad, heaven awaits. Those who did more evil than good go to hell (which has several levels corresponding to degrees of wickedness). There is an intermediate stage for those whose deeds weigh out equally.
This general principle is not absolute, however, but allows for human weakness. All faults do not have to be registered or weighed forever on the scales. There are two means of effacing them: confession and the transfer of supererogatory merits (similar to the Roman Catholic "Treasury of Merits"). The latter is the basis for Zoroastrian prayers and ceremonies for the departed.
Zoroaster invoked saviors who, like the dawns of new days, would come to the world. He hoped himself to be one of them. After his death, the belief in coming saviors developed further. He also incorporated belief in angels and demons.
Zoroaster's ideas of ethical monotheism, heaven, hell, angelology, the resurrection of the body, and the messiah figure have notable parallels in Judaism, Christianity, and Islam.