Abd al-Qahir al-Baghdadi

Abu Mansur Abd al-Qahir ibn Tahir al-Baghdadi (died 429 AH / 1037 CE) was a scholar of the Shafi'i school of Islamic law (madhhab) and a theologian of the Ash'ari school — the tradition of rational defence of Sunni creed founded by al-Ash'ari. He is best remembered as a heresiographer, a writer who catalogues religious sects.

Though called "al-Baghdadi" after Baghdad, where he is traditionally said to have been born around 980 (his birth date is not recorded and is only an estimate), his life's work belonged to Khurasan in the east. Sources report that his father, a man of means, brought him while young to Nishapur, then a great centre of learning. There he became the leading student of the Ash'ari master Abu Ishaq al-Isfarayini, and on his teacher's death succeeded him, teaching at the mosque of Aqil. Among those reported to have studied with him are the traditionist al-Bayhaqi and the Sufi writer al-Qushayri. He was also active in arithmetic, writing mathematical treatises.

His most influential book, al-Farq bayn al-firaq ("The Difference Between the Sects"), is built around a widely-circulated hadith that the community would split into seventy-three groups, only one "saved." Writing from a firmly Sunni, Ash'ari position, he describes the views he counts as errant and sets out the creed he holds to be correct; readers should note this is a partisan polemic, not a neutral survey. Late in life, amid Turkmen unrest, he reportedly left Nishapur for the town of Isfarayin, where he died and was buried near his teacher.