In the religious history of Iran the Saljuq period is particularly interesting, for it is the period of the Ismā‘īlīs. As the Ismā‘īlīs movement is treated in another part of the book, this chapter will be chiefly devoted to the three main aspects of religious life in Iran during this period: the development of Sunnism, the ferment of Shī‘ī ideas, and Sūfism. Chronologically the Saljuq epoch in Iran extends roughly from the tenth to the twelfth centuries; obviously in this chapter we cannot always keep exactly within these limits.

If we realize that in the years from the death of Ash‘ari (935) to that of Ghazālī (1111) the entire theological system of Islam found its final systematization; that it was also the period of Nizām al-Mulk's Siyāsat-Nāma and of extremely interesting Shī‘ī–Sunnī polemics; and finally that in the twelfth century the oldest Shī‘ī–Sunnī(fraternities) were organized, some of the first great Muslim theological universities were founded, and the poet Nizāmī lived (1141–1209/13): realizing these facts, we can easily see the importance of the Saljuq era. Though not one of the most original, it is certainly one of the most formative epochs in the cultural history of Iran.