Zoroastrianism's most remarkable feature is the dualism between the good god Ahuramazda and his rival, the Evil Spirit or The Lie. This translation of one of the key texts, Yasna 30.1-6, 8-9, was made by Mary Boyce.
[Y30.1-6] Truly for seekers I shall speak of those things to be pondered, even by one who already knows, with praise and worship for the Lord of Good Purpose, the excellent Wise One, and for Truth. [...]
Hear with your ears the best things. Reflect with clear purpose, each man for himself, on the two choices for decision, being alert indeed to declare yourselves for Him before the great requital.
Truly, there are two primal Spirits, twins renowned to be in conflict. In thought and word, in act they are two: the better and the bad. And those who act well have chosen rightly between these two, not so the evildoers. And when these two Spirits first came together they created life and not-life, and how at the end Worst Existence shall be for the wicked, but the House of Best Purpose shall for the just man. Of these two Spirits the Wicked One chose achieving the worst things. The Most Holy Spirit, who is clad in the hardest stone, chose right, and so do those who shall satisfy Ahuramazda continually with rightful acts.
The daevas indeed did not choose rightly between these two, for the Deceiver approached them as they conferred. Because they chose worst purpose, they then rushed to Fury, with whom they have afflicted the world and mankind.
[Y30.8-9] Then when retribution comes for these sinners, then, Mazda, Power shall be present for Thee with Good Purpose, to declare himself for those, Lord, who shall deliver The Lie into the hands of Truth. And then may we be those who shall transfigure this world. O Mazda and you other Lords, be present with support and truth, so that thoughts may be concentrated where understanding falters.