1- At the advent of the Sasanian period, Opposition between Zoroastrianism, Sasanian official and patronized religion, and Semitic religions, specifically Christianism, which traditionally rely on their holy books, compelled the clergies to give up their mere reliance on oral tradition and commence writing down the sacred book “Avesta”. This radical change very soon required them to generate exposition and exegesis, namely, Zand and subsequently commentaries on both Avesta and Zand which led to the prosperity of religious and juridical and consequently other types of Middle Persian literature. Furthermore, Compiled Avesta in that time not only contained religious subjects but also covered philosophy, logic, rhetoric, and even natural sciences such as medicine and astronomy which shows that everything was interpreted in connection with religion and consequently required explanatory treatises. Moreover, Sasanian kings’ exorbitant sponsorship of Zoroastrianism brought the priesthood endless political and financial power which enabled them to sponsor religious schools and consequently promoted composition of religious works more than before.
2- After the downfall of Sasanian dynasty, Zoroastrianism’s deprivation of administrative power, the threat of religious conversion, the decrease in the number of priesthood applicants and defect in the body of the Zoroastrian clerical system not only led to the expedition of religious texts’ copying but also compelled the prominent priests