Zoroastrianism is sometimes taught to be the first theistic religion. This depends on how figures like Abraham and Moses are understood. Nevertheless, Zoroaster did seek to reform the Persian polytheism of his day by teaching that there is only one God.
The particular themes that comes through are an emphasis on good and evil as due to the conflict between two created spirits, one good and one evil, which nevertheless find some kind of unity in God. Humans have free will to choose how to live, and after their death their deeds are judged. If a person has more good than evil deeds they go on to heaven, but if not they are cast into hell.
As a philosopher my interest is in thinking through this idea of free will (see my post on this topic) and the absence of redemption in Zoroastrianism. That is, humans are judged entirely by their works, but for those with more good than evil, there is no redemption needed for what evil they did. Although Zoroastrianism places a strong emphasis on the justice of God and the final judgment, what actually comes out is that God overlooks the evil done by persons who had more good.
However, if God is perfect in justice (a belief that is important to Zoroastrianism) then evil done by anyone, no matter how "small" (as if evil can be measured this way), is an infraction of justice and requires atonement/payment. Zoroastrianism does not have anything to say about redemption and atonement.
Nietzsche used the figure of Zarathustra (either another name for Zoroaster or a later reformer in Zoroastrianism) in his famous "God is dead" proclamation. When Zarathustra proclaims God to villagers, and they don't heed his call, Nietzsche portrays him as concluding that God is dead (no longer relevant for modern man). Nietzsche probably picked this figure because of the belief that he was the oldest theistic teacher, and because he was a reformer of polytheism.
Again, whether or not Zarathustra was the oldest theist depends on how one undersands other figures like Abraham and Moses. Some have claimed that the Jews got monotheism from the Zoroastrians, and that Christians incorporated Zoroastrian beliefs. However, the emphasis of both Judaism and Christianity on the need for redemptive atonement makes this claim false. Its seems possible that it is just the reverse, that influence from Judaism may have been behind Zoroastrianism. It is also possible that theistic belief arose independently in both religions due to some use of natural theology to understand that only God is eternal.
