The chief scripture of the Zoroastrians is the Avesta, composed of several sections that were not committed to writing until about the 4th or 5th centuries CE. The Gathas, or hymns, are the only scriptures attributed to Zoroaster, believed to be authentic sayings that survived from sermons he gave in the court of King Vishtaspa. Other sections include the Vendidad (statutes regarding purity), Siroza, Yashts (hymns to each of 21 deities), and Hadhoxt Nask (Sayings). The Zand (or Zend) Avesta refers to a compilation of holy texts committed to writing primarily in the 9th century CE plus the life and legends of the prophet, doctrine, law, science, and other writings. After successive waves of destruction by Arabs, Turks, and Mongols during the Islamic period, only about a quarter of its original material has survived. Zoroastrian sacred texts are written principally in Avestan and Pahlavi, two dead liturgical languages.
Although Zoroastrianism shares a common linguistic and theological heritage with the founders of Vedic Hinduism, its link to the Jewish-Christian tradition is far more significant. Having developed in the Middle East during the era when Judaism and Christianity were taking shape not far away, it is not surprising that similarities abound, but the question of who influenced whom is difficult for scholars to answer. If one accepts the earlier dates for Zarathustra, (c. 1500-1000 BCE), he could be seen as a forerunner of the monotheistic impulses of Akhenaten and Mose. At the very least, he contributed to the belief that each human being has the freedom to choose to live an actively virtuous life in the struggle of good against evil.
The Judaic and Christian beliefs in the appearance of a savior or messiah who would signal the end of the world and the beginning of a new creation; the division of humanity into good and evil at the Last Judgment; and the subsequent reunion of body and soul, with life everlasting for the righteous, have all been attributed to the influence of Iranian religious thought. The Jewish and Christian concept of the eternal flame burning before the altar may be derived from a similar Zoroastrian practice. (Zoroastrians are still known for their worship of the sacred fire, symbol of illumination — fire being considered the Son of Ahura Mazda and the highest gift of the creator, since it allowed great advances in culture.) And the Christian concept of the Holy Spirit as the executor of God’s will on earth seems to come directly from the Spenta Mainyu of Zarathustra, although some observers identify the Spenta Mainyu instead with the Logos, or creative agent, of Zeno, Philo, and the Gospel of John, where it refers to Jesus Christ.