Avesta
The sacred textual corpus of Zoroastrianism, the pre-Islamic religion of Persia founded by the prophet Zarathustra (Zoroaster), composed in Avestan, an ancient Iranian language related to Vedic Sanskrit, over a very extended period probably stretching from the second millennium BC (for the oldest hymns) to the early centuries of the common era, when the text was fixed and compiled into its present canonical form under the Sasanian dynasty. The oldest and most sacred core of the Avesta is the set of seventeen hymns called the Gathas, traditionally attributed to Zarathustra himself and composed in an archaic Avestan notably different from the rest of the corpus; they set out the fundamental doctrine of Zoroastrianism, an ethical and cosmic dualism according to which the universe is the stage of a struggle between the principle of good and truth, personified in the supreme god Ahura Mazda, and the principle of falsehood and chaos, personified in Angra Mainyu (Ahriman) — a struggle in which every human being, endowed with free will, must actively choose which side to stand on through their thoughts, words and deeds, summed up in the famous ethical triad 'good thoughts, good words, good deeds'. The rest of the Avestan corpus includes the Yasna (the principal liturgical text, which incorporates the Gathas), the Visperad (liturgical supplements), the Vendidad (a code of ritual purity and laws against the forces of evil) and the Yashts (hymns dedicated to lesser deities and entities of the Zoroastrian pantheon). Much of the original text was lost during the invasions and persecutions that followed the Arab conquest of Persia in the seventh century, and what survives today represents only a fraction of the original canon, transmitted and preserved chiefly by the Parsi community of India, descended from Zoroastrians who emigrated there fleeing Islamic persecution. The Avesta remains the foundational text of one of the oldest monotheistic religions still practised in the world.