No figure marks the fault line between the two great branches of Islam more sharply than Abu Bakr, and the difference is not a quarrel about his character but about legitimacy itself.
For Sunni Muslims he is the first of the Rashidun, the four Rightly-Guided Caliphs, and after the prophets the most excellent of human beings, the rightful successor, chosen by the community's consensus, whose brief reign saved Islam at its most vulnerable hour.
Shia Muslims hold that authority had been designated by the Prophet to Ali, his cousin and son-in-law, and that the succession at Saqifa set that designation aside. In the Shia account the caliphate of Abu Bakr is therefore a usurpation, and his reign is remembered through its conflicts, above all the dispute in which the Prophet's daughter Fatimah was refused her claim to the estate of Fadak, an episode of lasting grievance. Some currents of Shia piety have gone as far as ritual repudiation of him, which Sunni tradition regards as a grave offence against a revered companion.
The disagreement is thus foundational rather than incidental: it concerns whether legitimate authority passes by the community's choice or by the Prophet's designation, and it has structured Islamic history from the first days after his death.