Oral traditions held a special significance in pre-Islamic Iran and religious and literary works were transmitted by word of mouth for centuries. For instance, the Avestā only came to be written down in the Sassanian period after centuries of oral transmission. Some of the main causes that prompted the Zoroastrians to put their religious books into writing during the Sassanian period were, first and foremost, to protect them against the rapid spread of Islam and, secondly, to respond to the criticism that the followers of other scriptural religions like the Christians leveled against them. Besides making reference to the oral literature of the Median and Sakan languages of yore, some written literary texts in the Avestan and Old Persian have also survived in the ancient sources. Substantial literary works belonging to the Middle period of the two groups of Western Iranian languages (comprising Parthian and Middle Persian languages) and the Eastern Iranian languages (comprising Soghdian, Khwārazmian, Sakan, and Bactrian) are available today. Unlike the Zoroastrians, Māni and his followers gave great importance to penning down their religious books and some of their works in the Iranian languages (Middle Persian, Parthian, and Soghdian) have also survived to date.
Literature in Islamic Era
In its general sense, the “Fārsi” language is considered to be the same as the “Iranian” language and this term was used to indicate all the Iranian dialects by the writers of yore. Before the advent of Islam, the Fārsi language had spread throughout most parts of Asia. p Geographical Boundaries: Following the spread of Islam in Iran, the employment of the various Iranian dialects, including Dari, became the trend in places like Afghanistan, Pakistan, India, Iraq, Syria, Turkey, Oman, the Caucuses, China, and Central Asia. The Dari language was in use alongside the Pahlavi language from the Sassanian period. This language passed through various stages in the Islamic period and underwent major developments from the angles of form, structure, and contents and became the language of poetry and literature in the Islamic period in Iran and many literary works, both, in poetry came to be written in this language.