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.
[5] 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.
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.
"[12]Al-Shahrastani also explains the differences between Christians in Kitab al-Milal wa al-Nihal regarding the incarnation (tajassud): "They affirmed that God has three hypostases (aqanim).
They said that the Creator (may he be exalted) is one substance (jawhar), meaning by this what is self-subsistent (al-qa'im bi-n-nafs), not (what is characterized by) spatial location and physical magnitude; and he is one in substantiality, three in hypostaticity (uqnumiyya).
By the hypostases they mean the attributes (sifat), such as existence, life and knowledge, and the father, the son and the holy spirit (ruh al-qudus).