Quora Answer: Was Sayyidina ‘Ali ibn Abu Thalib (k.w.) Practicing the Same Islam as the Other Swahabah?

بِسۡمِ ٱللهِ ٱلرَّحۡمَـٰنِ ٱلرَّحِيمِ 

The following is my answer to a Quora question: “Was Sayyidina ‘Ali ibn Abu Thalib (k.w.) practicing the same Islam as Sayyidina Abu Bakr ‘Abdullah ibn ‘Utsman asw-Swiddiq (r.a.) and the rest of swahabah?  If yes, then how did Shi’ah deviate from Sunni Islam?” 

Why would the Islam of the son-in-law of the Prophet (s.a.w.) differ from Islam of the father-in-law of the Prophet (s.a.w.), when they both followed the same prophet, adhered to the same Qur’an, and worked together?  Slander of Sayyidina ‘Ali ibn Abu Thalib (k.w.) is an attack against the Prophet (s.a.w.), and an act of kufr. 

The dispute between the Sunni and Shi’ah branches of Islam arose out of a difference of opinion pertaining to the succession of the Prophet (s.a.w.), in ruling the nascent Madinan state.  It was a political difference first, before it became a theological one.  Those theological differences became pronounced much later, long after the time of the Khulafah ar-Rashidin.