Tāj al-Dīn Abū al-Fath Muhammad ibn `Abd al-Karīm ash-Shahrastānī (Arabic: تاج الدين أبو الفتح محمد بن عبد الكريم الشهرستاني; 1086–1153 CE), also known as Muhammad al-Shahrastānī, was an influential Persian historian of religions, a historiographer, Islamic scholar, philosopher and theologian. His book, Kitab al–Milal wa al-Nihal (lit. The Book of Sects and Creeds) was one of the pioneers in developing an objective and philosophical approach to the study of religions.

Very few things are known about al-Shahrastānī's life. He was born in 1086 CE A.H., in the town of Shahristān, (Khorasan, province of Persia) where he acquired his early traditional education. Later, he was sent to Nīshāpūr where he studied under different masters who were all disciples of the Ash`ari theologian al-Juwaynī (d. 1085). At the age of thirty, al-Shahrastānī went to Baghdad to pursue theological studies and taught for three years at the prestigious Ash`ari school, al-Nizāmiyya. Afterwards, he returned to Persia where he worked as Nā’ib (Deputy) of the chancellery for Sanjar, the Saljūq ruler of Khurāsān. At the end of his life, al-Shahrastānī went back to live in his native town, where he died in the year 1153.

Al-Shahrastani distinguished himself by his desire to describe in the most objective way the universal religious history of humanity.

This is reflected in his Kitab al-Milal wa al-Nihal (The Book of Sects and Creeds), a monumental work, which presents the doctrinal points of view of all the religions and philosophies which existed up to his time. The book was one of the earliest systematic studies of religion, and is noted for its non-polemical style and scientific approach. A French translation of the book by Gimaret, Monnot and Jolivet was sponsored by UNESCO (Livre des religions et des sectes. Peeters: 1986, 1993).

Al-Shahrastani's philosophical and theological thoughts manifested in his other major works, which include:

Although self-identified as an Ash'ari in terms of theology and a Shafi’i in terms of law, as can be seen in his books, Al-Milal wa al-Nihal, and Nihayat al-Iqdam fi 'Ilm al-Kalam, a few of his contemporaries accused him of covertly being an Isma'ili, and modern scholars believe that he was actually an Isma’ili practicing taqiya, or dissimulation, based on statements throughout his writings that correspond strongly with Isma’ili mysticism and its central doctrine of the Imamate.

Al-Shahrastani was amongst those attracted by the "new preaching" (da'wah jadidah) which Hasan-i Sabbāh, the Isma'ili da'i and founder of the medieval Nizari Isma'ili state, initiated. This preaching sought to spread the idea that humanity is always in need of infallible and divine teaching, which can only be provided by a divinely appointed guide. Al-Shahrastani tried to keep this a secret, but it was revealed by his student Al-Sam'ani. His works include a Quranic commentary that is infused with Isma’ili terminology, in which he hinted at his conversion by a "pious servant of God" who taught him how to find the esoteric (batin) meaning of the Quran. In his Kitab Al-musara'ah (Book of the wrestling match), al-Shahrastani criticizes Avicenna's belief that God is the involuntary necessitating cause of the world, and he also provides support for the Ismaili thesis that God is beyond being and nonbeing.

In Kitab al-Milal wa al-Nihal, al-Shahrastani records a portrayal of Christianity very close to the orthodox tenets while continuing the Islamic narrative: