Bahaiism, Buddhism, Christianity, Confucianism, Hinduism, Islam, Jainism, Judaism, and Sikhism, have their relevant sacred books. A closer look would, however, reveal that the conscious or unconscious founder of each religion or order had his or her inspired or thought-out message conveyed in person. Later the successors added much around the nucleus of the founding message and consequently produced a collection of writings, some of them in a different dialect or language. Some went even further. A linguistic and historical scrutiny of the collection, however, will reveal several layers of literature which could not but have taken almost a thousand years to materialize into an oral literature-oral because, like most of the sacred books of other religions, it was precisely and meticulously memorized and passed on by word of mouth through generations until its final reduction in writing.
Tradition says that it was put in writing in the very earliest times.
The collection suffered a disaster when Alexander of Macedonia invaded Iran 2321 years ago in 321 BCE, put an end to the Achaemenian empire, and devastated the royal treasuries in which the Avesta was reportedly kept. It may be noted that during the entire period of collecting, collating, and canonizing of the Avesta, Jews and Christians were also engaged in a similar move, and the present forms and orders of all sacred scriptures are the result of meticulous labor over centuries. Yet critical studies of all them continue to find new and sometimes startling points about their original texts, volumes, languages, styles, and the hands of those who have edited, at times interpolated, adulterated, added, and deducted to give the final forms to the scriptures before their canonizations. The Sassanian canon of the Avesta was divided into 21 volumes, called nasks in the Pahlavi language. The nasks were put into three categories of seven each.