Meanwhile, in the central caliphal lands, growing discontent with the emerging order crystallized in a multifaceted movement of opposition to the Marwānids. It culminated in the third fitnah (744–750), which resulted in the establishment of a new and final dynasty of caliphs, the ʿAbbāsids. Ever since the second fitnah, a number of concerned and self-conscious Muslims had begun to raise serious questions about the proper Muslim life and the Marwānids’ ability to exemplify it, and to answer them by reference to key events in the ummah’s history. Pious Muslims tried to define a good Muslim and to decide whether a bad Muslim should be excluded from the community, or a bad caliph from office. They also considered God’s role in determining a person’s sinfulness and final dispensation. The proper relationship between Arab and non-Arab Muslims, and between Muslims and dhimmīs, was another important and predictable focus of reflection. The willingness of non-Arabs to join the ummah was growing, but the Marwānids had not found a solution that was either ideologically acceptable or fiscally sound. Because protected non-Muslim groups paid special taxes, fiscal stability seemed to depend on continuing to discourage conversion. One Marwānid, ʿUmar II (ruled 717–720), experimented unsuccessfully with a just solution. In these very practical and often pressing debates lay the germs of Muslim theology, as various overlapping positions, not always coterminous with political groupings, were taken: rejecting the history of the community by demanding rule by Muhammad’s family; rejecting the history of the community by following any pious Muslim and excluding any sinner; or accepting the history of the community, its leaders, and most of its members.

In the course of these debates the Marwānid caliphs began to seem severely deficient to a significant number of Muslims of differing persuasions and aspirations. Direct and implied criticism began to surface. Al-Ḥasan al-Baṣrī, a pious ascetic and a model for the early Sufis, called on the Marwānids to rule as good Muslims and called on good Muslims to be suspicious of worldly power. Ibn Isḥāq composed an account of Muhammad’s messengership that emphasized the importance of the anṣār, the Yathribi tribes that accepted Muhammad, and by implication the non-Arab converts (from whom Ibn Isḥāq himself was descended). The Marwānids were accused of bidʿah, new actions for which there were no legitimate Islamic precedents. Their continuation of pre-Islamic institutions—the spy system, extortion of deposed officials by torture, and summary execution—were some of their most visible “offenses.” To the pious, the ideal ruler, or imam (the word also for a Muslim who led the ṣalāt), should, like Muhammad, possess special learning and knowledge. The first four caliphs, they argued, had been imams in this sense, but under the Umayyads the caliphate had been reduced to a military and administrative office devoid of imāmah, of true legitimacy. This piety-minded opposition to the Umayyads, as it has been aptly dubbed, now began to talk about a new dispensation. Some of the most vocal members found special learning and knowledge only in Muhammad’s family. Some defined Muhammad’s family broadly to include any Hāshimite; others, more narrowly, to include only descendants of ʿAlī. As the number of Muhammad’s descendants through ʿAlī had grown, numerous rebellions had broken out in the name of one or the other, drawing on various combinations of constituencies and reflecting a wide spectrum of Islamic and pre-Islamic aspirations.

In the late Marwānid period, the piety-minded opposition found expression in a movement organized in Khorāsān (Khurasan) by Abū Muslim, a semisecret operative of one particularly ambitious Hāshimite family, the ʿAbbāsids. The ʿAbbāsids, who were kin but not descendants of Muhammad, claimed also to have inherited, a generation earlier, the authority of one of ʿAlī’s actual descendants, Abū Hāshim. Publicly Abū Muslim called for any qualified member of Muhammad’s family to become caliph, but privately he allowed the partisans (shīʿah) of ʿAlī to assume that he meant them. Abū Muslim ultimately succeeded because he managed to link the concerns of the piety-minded in Syria and Iraq with Khorāsānian discontent. He played upon the grievances of its Arab tribes against the tribes of Syria and their representatives in the Khorāsānian provincial government, and on the millennial expectations of non-Arab converts and non-Muslims disenchanted with the injustices of Marwānid rule.

When in 750 the army organized and led by Abū Muslim succeeded in defeating the last Marwānid ruler, his caliph-designate represented only one segment of this broad coalition. He was the head of the ʿAbbāsid family, Abū al-ʿAbbās al-Saffāḥ, who now subordinated the claims of the party of ʿAlī to those of his own family and who promised to restore the unity of the ummah, or jamāʿah. The circumstances of his accession reconfigured the piety-minded opposition that had helped bring him to power. The party of ʿAlī refused to accept the compromise the ʿAbbāsids offered. Their former fellow opponents did accept membership in the reunified jamāʿah, isolating the People of the Shīʿah and causing them to define themselves in terms of more radical points of view. Those who accepted the early ʿAbbāsids came to be known as the People of the Sunnah and Jamāʿah. They accepted the cumulative historical reality of the ummah’s first century: all the decisions of the community and all the caliphs it had accepted had been legitimate, as would be any subsequent caliph who could unite the community. The concept of fitnah acquired a fully historicist meaning: if internal discord were a trial sent by God, then any unifying victor must be God’s choice.