Early in Islamic history, some Shi'i Muslims began to publicly denounce the first three caliphs as illegitimate, an act that was and is deeply offensive to the Sunni community who regard these men as icons of the early Muslim community.
An ancient religious divide is helping fuel a resurgence of conflicts in the Middle East and Muslim countries.
They believe that these individuals possess spiritual charisma, and assert that they rank just below the prophets because they are divinely inspired, not to produce new prophecy, but to understand the true or esoteric meaning of the Quran.
In their eyes, therefore, Muhammad's relatives and descendants were the proper leaders of the Muslim community and of the first four caliphs only the fourth, 'Ali, Muhammad's first-cousin and son-in-law, was legitimate.
The monotheistic profession of faith – "There is no God but God and Muhammad is the Prophet of God," 2. In Sunni eyes, these s did not possess, as some Catholic Popes claimed, infallibility in interpreting religious doctrine.
Sunnis view the first four caliphs, men who had known or were related to Muhammad, idealistically as the four "rightly-guided Caliphs," (632-661 CE) of an Islamic Golden Age, and most of them also accept the legitimacy of both the two later dynastic Caliphates: the Umayyads (661-750 CE) of Damascus and the 'Abbasids (750-1258 CE) of Baghdad and those who ruled individual Muslim countries afterwards.
How their rivalry is settled will likely shape the political balance between Sunnis and Shias and the future of the region, especially in Syria, Iraq, Lebanon and Bahrain.
Alongside the proxy battle is the renewed fervor of armed militants, motivated by the goals of cleansing the faith or preparing the way for the return of the messiah.
These Muslims take essentially the same attitude as fundamentalists in other religious traditions, ranging from Christians to Hindus.
Shi'i Muslims differ from Sunnis in that they not only revere Muhammad's family, but attribute unique religious insight to his relatives and descendants.