Reddit reply in r/AcademicQuran: evolution of sects within islam did shiism branch — January 26, 2025 | Ali Shukri Amin

Reposting an old comment:

All Islamic sects came from controversies. They are signified by their positions on issues of dispute. There's two major sources of these issues: pre-existing theological/philosophical issues or internal Islamic issues. I'm segregating issues that did exist in other religions or philosophy but that developed independently as internal issues.

The first "sect" was proto-Sunnism. However, it wasn't codified into a formal theology until later than other sects like the Qadiris, Khawarij, and Mu'tazilah. The reason is that a unique Sunni identity was defined in contrast to the "deviant" sects.

The first disputes were internal and had to do with the religious implications of succession. Namely, the caliphate after the death of the Prophet. Ali b. Abi Talib, the Ghadir Khumm, and Saqifah incident caused the first split - however this didn't crystallize into a formal sect until after Ali's death with some time.

The arguably very first sect that was distinct from proto-Sunnism was Kharijism (modern Ibadism). Sunnis claim this actually began during the life of the Prophet and that he warned against them. Kharijites disputed some of Uthman's political choices and Ali's arbitration after his killing. Sunnis claim that the khawarij killed both of these caliphs.

The succession issues became central theological doctrines for the Umayyad and Abbassid dynasties, both of which claiming legitimacy by being anti-Ali (pro-Uthman) or pro-Ali (family of Abbas). It wasn't uncommon for Ali to be cursed on Umayyad pulpits or Abu Bakr and Umar to be cursed as a formal litany by Shi'ites during these dynasties.

Following this, as the Islamic empires expanded, influences from Buddhism, Greek philosophy, and Christian discourse brought a lot of those ideas into the millieu. Muslim theologians debated and chose sides on practically every ancient philosophical issue presented to them and tried to reconcile their views with the Quran.

The first of these was the issue of predestination, something debated in Eastern and Western philosophy. The extremes were the Qadirites (total free will) and the Jabirites (compulsionists). Again, the Umayyad and Abbassid states took official opinions on these issues. You can look at Hassan al-Basri's epistle castigating the Umayyads for using Jabirism to justify excesses or the epistle upholding Qadr by Umar ibn Abd al-Aziz.

Jahm bin Safwan seems to have introduced issues about divine ontology and metaphysics from Greek-influenced Christian thinking. This is where we have discussions about God's Names & Attributes (one of the most contentious issues in Islamic theology) and the Divine nature of the Qur'an (Khalq al-Qur'an). These issues are still hotly debated today, over 1000 years later (despite the Quran and Sunnah explicitly discussing neither).

The Mu'tazilites were the first to develop a formal theological system to discuss all of these issues and reconcile it with the Sunnah/hadith. Sunnis always had much more of a monopoly on the Sunnah but until forced to respond to the Mu'tazilah on religious and political grounds did not develop a formal theology to address all of these external controversies.

Any intermediate level Aqidah text will give you a high level overview as to what the authors were most concerned about if read contrapuntually. Read Tahawi's Aqidah and just imagine the person Tahawi is refuting, take the exact opposite opinion in your head and you'll understand the full scope of discussion.

A pretty comprehensive book is William Montgomery Watts' "Formative Period of Islamic Thought." You can also look at the Oxford Handbook of Islamic Theology or Yasir Qadhi's PhD thesis. Kalamopod on Youtube is a good place to start.

I also recommend just reading the source texts in historical order.