The Avesta
The Avesta is a collection of the sacred texts of the Mazdaist (Zoroastrian) religion. Although some of the texts are very old, the term Avesta itself only dates to the second century CE.
It is widely believed that Zoroastrianism is the earliest monotheist religion and had a key influence on Judaism, Christianity and Islam.
The origin of the religion is ascribed to the prophet Zarathustra, who is commonly known in the Western world as Zoroaster, the Greek version of his name. Zoroaster is thought to have composed the Gathas, poems which were assiduously preserved by his followers through centuries of oral transmission.
Ahura Mazda is the beginning and the end, the creator of everything which can and cannot be seen, the Eternal, the Pure and the only Truth. Zoroastrian morality is summed up in the simple phrase, "Good Thoughts, Good Words, Good Deeds". Central to Zoroastrianism is the emphasis on moral choice; of life as a battle-ground between moral and immoral forces, represented by Spenta Mainyu and its satanic antithesis Angra Mainyu, the 'good spirit' and 'evil spirit' emanations of Ahura Mazda.
Zoroastrianism teaches many of the concepts found in the major Abrahamic faiths such as Heaven, Hell, the Last Judgment, Satan, prophecy, the coming of the Messiah, angels, and evil spirits. Predestination is rejected in Zoroastrian teaching. Humans bear responsibility for all situations they are in, and in the way they act to one another. Nothing in the Heavens and Earth has the power to force a being to do evil. Reward, punishment, happiness and grief all depend on how individuals live their life. Good befalls the people who do righteous deeds. Those who do evil have themselves to blame for their evil-doing.
Humans possess a great power. They can improve their way of living and the living conditions of others. This power is called Charitas. After death the person must walk through the Path to Judgement or Chinvat Peretum to bear responsibility for his or her actions when alive. There is a belief in heaven and hell in Zoroastrian cosmology but it is a little different than that of the Christian hell. The evil are sent to hell until the time when evil is finally defeated, at which time they will go through a purgation process, the "ordeal of molten metal", and then join Ahura Mazda and the saints. Thus, Zoroastrianism can be said to be a Universalist religion with respect to salvation.
The concept of Dualism plays a role when speaking of the Spenta Mainyu ("Holy Spirit") and the Angra Mainyu ("Evil Spirit"). These two have a constant battle, at the end of which the Holy Spirit will prevail by the power of Ahura Mazda.
Kordeh Avesta (english version)
YASNA: Sacred Liturgy and Gathas/Hymns of Zarathushtra