The Bundahishn, meaning primal creation, is a major Zoroastrian cosmological text composed in Middle Persian, drawing on and synthesizing older Avestan material now lost in its original form, to describe Zoroastrian cosmogony, the cosmic struggle between the forces of Ahura Mazda and the destructive spirit Angra Mainyu, and the structure of the created world in extensive systematic detail, making it one of the most important surviving sources for reconstructing developed Sasanian-era Zoroastrian theology.
Middle Persian also produced the Zand, an extensive genre of interpretive commentary and translation on Avestan scripture, since by the Sasanian period few if any Zoroastrians could understand Avestan directly without such interpretive assistance, a situation historians of religion compare to the role Aramaic Targums played in helping Jewish communities access Hebrew scripture once Aramaic had displaced Hebrew as an everyday spoken language. Zoroastrian priests in both Iran and among the Parsi community of India continued producing and studying Pahlavi religious texts for centuries after the Arab conquest, preserving a substantial theological literature that modern Zoroastrian studies scholarship continues to draw upon heavily today.