Muĥammad ibn Ábd al-Karīm ibn Aĥmed Abu’l Fat’ĥ – famously known as Al-Shahrastānī is the author of the book Al-Milal wa’n Niĥal; and in my opinion, this is the best book written on the subject [of heresiology]. Even though, Ibn Ĥazm’s book1 is more extensive than this one, it is haphazard and lacks organization. Morever, there are [opinions] denigrating imams of Ahlu’s Sunnah; and in a number of places, he attributes such opinions to Ashárīs, which they do not hold.2 And on top of this, Ibn Ĥazm himself does not have a proper understanding of the science of kalām, as described by the authorities [in that science]. 

Nihāyatu’l Iqdām is also Shahrastānī’s work; he has written books other than these two. He was an Imām, and a foremost authority and researcher in this science.  He was a master of Fiqh, Usūl and Kalām. Aĥmed al-Khawafiyy was his teacher in Fiqh. He was the student of Abū Naşr, the son of Imam Abu’l Qāsim al-Qushayrī the grandmaster, in the twin sciences of Usúl [al-Fiqh] and Kalām. He also studied Kalām under the teacher Abu’l Qāsim al-Anşārī.

Ibn al-Samáānī writes: He arrived in Baghdād in the year 510 AH, and stayed for three years. He held lectures and was well received by the public. In Nishapur, he attended the circle3 of Abu’l Ĥasan Álī ibn Aĥmed al-Madīnī and others. I asked him about his date of birth, and he said that it was 479 AH. He passed away in 548 AH.

The above is cited from Ibn al-Samáānī’s Al-Dhayl4 and also narrated by Ibn Şalāĥ in his Ţabaqāt. I have two [different] copies of Al-Dhayl and I do not find [in it] anything more than what I have cited above; except that he narrates a ĥadīth from him and mentions two authenticated anecdotes; and he5 said that he heard these [from him] in a discourse: ‘I was asked in a gathering in Baghdad about Mūsā şallAllāhu álayhi wa sallam ; and I replied: Mūsā şallAllāhu álayhi wa sallam turned right and left and he couldn’t see anything of comfort or a dweller retire; and he saw, by the side of the mountain, a fire.

This is summarized from Al-Dhayl of Ibn al-Samáānī.

—-

In Tārīkh6 of our Shaykh Al-Dhahabī, he says that Ibn al-Samáānī mentioned that he [Al-Shahrastānī] was accused of leaning towards the Ahl al-Qilāá,7 that is the Ismaýīlīs; and that he [was involved in] preaching and aiding their heresies. And he said in Al-Taĥbīr,8 that he was accused of being a [closet] heretic9 and was inclined towards their [Ismaýīlī] beliefs; and that he was a fanatic Shiáh. [summarized from Dhahabi’s note].

As far as Al-Dhayl is concerned, there is nothing of the kind10 in it; rather, this is found in Al-Taĥbīr. I do not know where Ibn al-Samáāni got this from, because the books of Abūl Fat’ĥ [al-Shahrastānī] are contradictory to this accusation.  I think these are inserted in Ibn al-Samáānī’s book; if not, why does he not mention the same in his [other] book, the Dhayl?11

Nevertheless, a similar statement was said by the author of Al-Kāfī: ‘if he did not have a ravaged áqīdah, nor an inclination towards heretics, he would be considered as an Imām in Islam.’ And he said: ‘We had discussions and debates; and he was foremost in supporting the madh’hab of the philosophers and absolving them.’ That is, Al-Khawarizmi said [the above].

Ţabaqāt al-Shāfaýiyyah al-Kubrā; Imām Tājuddīn al-Subki. Volume 6, Pgs 128-130 /#653

———————————————————————————–

Footnotes:
[1] Which is also commonly known as Milal wa’n Niĥal and its full name is Al-Fişal fi’l Milali wa’n Niĥal
[2] Lit. ‘which they are absolved of’
[3] Lit. ‘he heard from’
[4] The Appendix
[5] Ibn al-Samáānī
[6] Tārīkh al-Islām, The History of Islam, by Imām Dhahabi, the author of Siyar Aálām an-Nubalā, Tadhkiratu’l Huffāž and other works.
[7] probably a reference to the mountain fortress of Alamut, the stronghold of Nizari Ismayilis.
[8] by Ibn Al-Samáānī
[9] Mulĥid
[10] as mentioned by Dhahabi
[11] Note that Subki has commented earlier that he has two copies of Dhayl and neither has this information as claimed by Dhahabi.