The main theological difference between Sunni and Shia Islam centers on leadership succession after Prophet Muhammad. Sunnis believe the Prophet’s companions chose his rightful successors, starting with Abu Bakr, while Shias hold that Ali, the Prophet’s cousin and son-in-law, was divinely appointed as his successor. This divergence affects religious authority and interpretation, with Sunnis following the consensus of the Prophet’s companions and Shias emphasizing the guidance of imams descended from Ali.