Armstrong writes that the theological questions about the nature of political power that had been raised by the Fitnahs were as profound to the Muslim faith as the early Christological debates had been to Christianity. Several religious movements that opposed the absolutism and exorbitance of the Caliphs emerged, and a new jurisprudential culture aimed at standardizing legal interpretations of Islamic Law (Shariah) motivated the creation of a new class of legal scholars called ulama, who helped to integrate spiritual teachings into the legal landscape of the Caliphate.

After being defeated by European forces at Poitiers in 732, the Umayyad Caliphate retreated to Spain, which would remain the farthest reach of the empire. The final Umayyad Caliphs began prioritizing religious policies rather than economic ones; Umar II (r. 717-270), for example, began encouraging dhimmis to convert to Islam. Armstrong argues that although these religious policies resulting in religious contentment amongst the population, the political infrastructure of the Caliphate began to crumble. At the same time, a schism within the religion occurred, with the followers of Ali branding themselves “Shia.” General discontent amongst former dhimmis who were now being treated as second-class Muslims, and the destabilization caused by the schism, allowed the Abbasid family to seize control and establish their own Caliphate.

The first Abbasid Caliphs consolidated power by massacring as many Umayyads and Shias as they could find. They established their capital in Baghdad and began enacting policies that destroyed the ethno-religious hierarchies that had been put in place by the Umayyads. Influenced by the Sassanid monarchs of pre-Islamic Iraq, the Caliphs lived in isolated luxury and flirted with absolutism. Meanwhile, the Shias began treating the descendants of Ali as spiritual leaders (Imams), rather than political ones. The Imams practiced a mystical form of Islam that appealed to the elite classes, while the Sunnis developed a more pragmatic form of worship based on jurisprudence. Legal scholars (ulamah) innovated the concept of Shariah, an Islamic Law grounded in the religious texts. In the ninth century, a civil war ensued over a dynastic dispute. As Shariah ideology became more coherent and more populist, the divine authority of the Abbasid Caliphs was increasingly dubious.

During the Abbasid years, a complex culture of philosophical and theological Islamic study emerged. These new schools of Islam were only accessible to the educated elite, who believed that their own ideas would be misinterpreted by uneducated people. In response to the hyper-pragmatic jurisprudential culture that was developing in the public sphere, the esoterics incorporated mystical themes into their practice of Islam.

The theology of the Shias, who continued to believe that the rightful caliphs were the descendants of Muhammad, was radically transformed when the 12th generation Imam disappeared under mysterious circumstances in 874. Shias began calling him the Hidden Imam, transforming him into a messianic cult figure, believing that he would return to the earthly realm and initiate an era of divine justice. This school of Shia Islam would later be called Twelver Shiism, and the majority of Shia Muslims in the present day are Twelver Shias. Other Shias believed that the line of legitimate Imams had ended much earlier, and called themselves Seveners or Ismailis.

The importation of ancient Greek and Roman philosophical texts had a profound effect on the esoteric movements as well. Thinkers like al-Kindi (d. 870) and al-Farabi (d. 950) believed that philosophy offered the devout a route to a more transcendent form of Islam, less constrained by the worldly and more concerned with the deepest part of the soul. This kind of thinking was popular amongst Sunni mystics (sufis), who were just as concerned about whether the Caliphate was continuing the spiritual mission of Muhammad as the Shias were. When the Abbasid Caliphate eventually crumbled, Armstrong argues that everyday Muslims would be spiritually liberated.