hi! This question may be worth x-posting to /r/AskReligion or a more specialist Islamic sub, but there have been many posts here on related topics - take a spin through these for source references/recommendations

• The Sunni / Shi'a split seems grounded in a fairly innocuous and ancient dispute. Leaving aside their respective beliefs about the succession, what events have led to the sharp and violent conflict between these two groups? <-- includes links to several more

• thread in Do you hold any contrarian views in your field of study? If so, please outline them here, and also describe the reactions of fellow historians.

• How did the Sunni Shia split become so uneven? How did Iran become the hub of Shia activity and did these divisions lead to Iran's current borders? - not quite on topic, but you may find it interesting