al-Shahrastani

Abu al-Fath Muhammad ibn 'Abd al-Karim al-Shahrastani (479-548 AH / 1086-1153 CE), also called Taj al-Din, was a theologian and historian of religions from Khurasan, in the Persian-speaking east of the medieval Islamic world. He is best known for Kitab al-Milal wa-l-Nihal ("The Book of Religions and Sects," written around 1127), an ambitious survey that classifies Muslim groups, then other faiths and the doctrines of the Greek and Islamic philosophers. It is widely regarded as the most important Muslim heresiography (catalogue of sects) of the premodern period and is still read as a major source for ideas it sometimes preserves in no other place.

He studied at Nishapur with masters in the tradition of the Ash'ari school of kalam (rational theology), and for roughly three years he taught at the famous Nizamiyya college in Baghdad. He also served, by report, as a deputy in the chancellery of the Seljuk ruler Sanjar before returning to his home town, where he died.

Outwardly al-Shahrastani is associated with Sunni Ash'arism, and his theological summa Nihayat al-iqdam works within that tradition while criticizing parts of it. A number of modern scholars, however, argue from his later writings that he held Isma'ili Shi'i sympathies concealed by taqiyya (prudential dissimulation). This remains genuinely disputed: it is a scholarly hypothesis, not a settled fact, and other specialists read the same evidence as academic engagement rather than personal allegiance.