There is a strange persistence in many Western studies of the religious history of Islamic Iran to be concerned almost solely with small sects and extremist religious movements. A notable feature of the Islamization of Iran that is reflected in the work of the Persian scholars, who were concerned with the religious sciences is strong opposition to the Zanādiqa and similar heterodox groups. The sciences dealing with the Quran are divided traditionally into those dealing with the recitation of the Book and those dealing with commentary upon it and the meaning of its content. The Quran itself provides in many places intellectual evidence and demonstration for its arguments and is the example for all later developments of Islamic religious sciences in which rational arguments are provided for articles and principles of faith. The efforts of Persian scholars in helping to lay the foundation of many of the Islamic sciences during the early period continued into the Saljuq and Timurid periods.

This chapter discusses the philosophy and cosmology in Persia from the rise of Islam to the Saljuq period, which is almost synonymous with studying the first phase in the development of Islamic philosophy and cosmology itself. The Persian translators and the whole class of secretaries who cultivated philosophy were important in creating the new style of philosophical prose in Arabic. The early history of Islamic philosophy and theology during the 3rd/9th century is connected with the cities of Baghdad, Basra and Kufa, in all three of which the Arab and Persian elements were mixed, such that it is often difficult to separate them. The written record of Islamic philosophy begins with the "philosopher of the Arabs" Abū Ya‘qūb al-Kindī, who wrote extensive treatises in Arabic on philosophy and the sciences during the 3rd/9th century, relying most of all on the translations of Syriac scholars as well as of course on Islamic sources.

In the religious history of Iran the Saljuq period is particularly interesting, for it is the period of the Isma'ills. This chapter devotes to the three main aspects of religious life in Iran during this period: the development of Sunnism, the ferment of Shi'I ideas, and Sufism. The importance of the Saljuq period in the religious history of Iran lies in its formative richness, expressed in various directions of thought: first, Ash'ari Sunnism reached its final systematisation in the great synthesis of Ghazali. Secondly, Sufism was first organized into great brotherhoods, and important schools were created. Thirdly, the philosophy of Suhravardi Maqtul opened up new paths to Iranian theosophical speculation. And fourthly, Shii ferment pullulated in Iran in the double aspect of Isma'Ilism, with its highly interesting esoteric theology, and Twelve Imamism, which, though now comparatively weak, created a wide network of propaganda centres, during the Saljuq period.