Zoroastrianism  Zoroastrianism is the oldest religion in the world that it is still practiced in its original form, and was a significant influence on later religions. Starting in Central Asia around 2000 BC, it became the religion of three great empires (Achaemenid, Parthian, Sassanid), but after the Arab conquest of Persia it lost its Imperial status, and is now practiced only by small communities scattered throughout the world. Nevertheless, its status as the world's oldest religion and the first monotheism, as well as its influence on religions such as Judaism, Christianity, Islam, and Buddhism, makes it an important religion to understand. Here I will talk about the theology, moral code, mythology and rituals of Zoroastrians.

Theology:
The primary characteristic of Zoroastrian theology is dualism. In Zoroastrianism, there are two gods- Ahura Mazda (or Ohrmazd), who is entirely good, and Ahriman, who is entirely evil. The purpose of our lives is to follow Ahura Mazda, and reject Ahriman- Ahura Mazda is incarnated in light, fire and truth, and Ahriman is incarnated in darkness, filth and lies. Besides these two gods, there are also many lesser divine beings- Ahura Mazda has 7 Amesha Spentas (Kindly Spirits) to help him, and each person also has their own Fravasha (Angel) who reminds them of their duty to follow Good and reject Bad. Ahriman, on the other hand, has 7 Daivas (Demons) to help him, and there are many creatures that are creations of Ahriman who also help him.

Moral Code:
For a Zoroastrian, the worst sin one can commit is to tell a lie- it is better to murder someone and tell the truth than not to murder and to lie, because murder is an insult to humanity, but lying is an insult to god. There are 2 forms of lying: lying through ignorance and lying through malice, and both are evil. Lying through malice is worse, because it is done with intent to harm, but lying through ignorance is also bad. This is why the two most important things for Zoroastrians are mental discipline and education- mental discipline will prevent lying through malice, and education will prevent lying through ignorance. The other important aspect of Zoroastrian morality is purity: the mind, body and elements of the world (earth, fire, water, air) must be kept clean. how they do this is explained more in the Rituals section.

Mythology:
For Zoroastrians, almost everything about the world can be explained by their Creation and Apocalypse myths, with the addition of the Gayomart/Saoshyant myth, so I will tell these three myths.
Creation:
In the beginning, the universe was empty, except for Ahura Mazda, Ahriman, the 7 Amesha Spentas and 7 Daivas. Then Ahura Mazda created the world, the stars, plants, animals, the sun, the moon, everything. The first man was Gayomart, and everything was perfect, because it had been created by the perfect being. Ahura Mazda himself could not enter creation, because it was he who had created it (can a watchmaker go into his own watch?). But Ahriman could enter it, and he did, along with his Daivas. Ahriman created all the bad things in the world: sickness, death, scavengers, any insects, and the impulse to do bad things. We have morality and justice and conscience because Ahura Mazda created us with them, but we do bad things because Ahriman gave us the impulse to do so. Also, Ahriman can help his followers in this world, but Ahura Mazda cannot- this is why evil is so often victorious over good. In an attempt to counter Ahriman, Ahura Mazda sent his Amesha Spentas into the world to ensure the survival of Good in the world, but he knew that creation was ruined. So he created Time, and the end of Time. At the end of Time, the world would be destroyed, and everything in it- including Ahriman and the Daivas. Ahura Mazda, because he exists outside the world, will not be affected by the end of time, and when this world is destroyed he will create a new one, which will be even more perfect, because Ahriman and the Daivas will no longer exist.
Apocalypse:
At the end of Time, the world will be destroyed, and all the souls of this world must cross into Ahura Mazda's new perfect world. To do so, however, they must cross the River of Purity- the water of this river will burn away the sins of the soul. This means that if you have committed many sins, your soul will be full of them, and the water will feel like burning metal, but if you have committed few sins, your soul will already be pure, and the water will not hurt at all. At the end, however, every soul will emerge pure and free from sin, as well as the impulse to commit sin, because Ahriman and the Daivas have been destroyed. At this time the new world will begin, populated only by pure souls.
Gayomart/Saoshyant:
As we saw in the Creation myth, Gayomart was the first man, and, because he was born before Ahriman's entry into the world, he was pure and free from sin. Zoroastrians believe that, because he was a perfect creation, he did not die- his body died, but his essence survived at the bottom of a lake. Some years before Apocalypse, a virgin woman who has committed no sin will go swimming in the lake, and emerge pregnant by Gayomart- her son will be the Saoshyant, the second pure person and the only one since Ahriman's entry into the world. He will open the eyes of the world, and announce the coming of Apocalypse, when he will lead the souls of the world through the River of Purity and into the new world.

Rituals:
the Avesta:
Unlike many other religions, Zoroastrianism has no true equivalent of the Torah/Bible/Qur'an/Sutras. They do have a holy book, the Avesta, but they treat it very differently. This is because all copies of it were destroyed in the 300s BC by Alexander the Great (Alexander the Defiler for Zoroastrians), and when they were allowed to write it down again 200 years later, no-one could remember all of it properly- hence the Avesta, while a useful thing, is not the word of God but the word of man for Zoroastrians, and it may be wrong.
Initiation:
Zoroastrians also believe that religion is a matter of choice, and no-one can be truly Zoroastrian unless they have gone through the initiation ceremony, which can only be performed for people older than 15 years old- no-one is born Zoroastrian, unlike in Christianity, Judaism, and Islam, in which infants are baptised into the faith before they can even speak. 
Fire-worship:
Zoroastrians are commonly referred to as 'fire-worshippers', because the focus of their temples is a sacred fire. However, the fire is not actually worshipped- the fire is only a symbol of Ahura Mazda. Zoroastrians concentrate on fire when they worship Ahura Mazda, but this does not mean that they are worshipping the fire itself.
Waste Disposal:
Since Zoroastrians believe that the elements are sacred and should be kept pure, they try not to put waste into the earth, water or fire. Things like dead bodies, waste food etc. they feed to vultures, wolves, insects and other things that eat it, since they are creatures of Ahriman, and because they are already evil it is no sin to defile them. Things that are not eaten by animals or insects Zoroastrians try to avoid using, and if they must use it they use it until it falls apart.

If anyone has any interesting information, or questions, please ask!