The beginning of the Sunni / Shia divide was originally political......how the leader over the muslim people was to be chosen.
Sunni position: The best person to lead is voted on by a council of elders.
Shiite position: The leader must be a direct bloodline descendant of Prophet Muhammad.
Over the years Shiite hereditary leadership has morphed into the theocratic rulers in Iran and southern Iraq called the Ayatollah.
The Ayatollah is much like the Catholic Pope in that he speaks for God. The difference is the Ayatollah's words must be obeyed without question.
The Sunni's on the other hand, have no religious hierarchy; and are under no obligation to obey any Imam or religious leaders rulings or fatwas.
The animosity today between Sunni and Shia today mainly has to do with religious doctrine and practice.
In order to separate themselves from the majority Sunni's; .....the Shiites over the centuries have developed a heretical cult version of Islam that is extremely offensive to Sunni muslims; and full of blasphemy and slander towards Muhammad's wives and companions.
I hope this helps......
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