The Formation of Islam begins with the setting, with the Near East and pre-Islamic Arabia, and with Judaism, Christianity, Zoroastrianism, Manichaeism and paganism. "Whatever the precise contours of the relationship between Judaism and emergent Islam, it is certain that the new faith can only be understood against the background of Jewish messianism and, more broadly, of the religious turmoil which characterized the Near East at the end of late antiquity." This broad view of religion, as suggested by the subtitle, extends into the next two sections, which both contain chapters on the non-Muslim communities. Not only did Islam evolve in an existing religious context, but there was a long period in which influences ran in both directions: "by the end of the Umayyad period, Zoroastrianism was hardly moribund, and even in the cities, the Muslim community may have represented only a small minority of the population".
"The Emergence of Islam, 600-750" describes the origins of the Muslim community and the development of Islam during the Umayyad period. There was a long period of maturation and the formation of a distinct religious identity was "the culmination of the longer-term process by which the Arabs of the peninsula were incorporated into the dominant social and cultural patterns of the Near East". Many aspects of Islam now considered fundamental took some time to develop, or to settle into their familiar forms. For the early period there are problems with sources which were mostly written in the context of later debates and divides.
Sectarianism was present from the start, notably in a diverse range of Khariji rebellions driven (at least ostensibly) by disapproval of the behaviour of Uthman and subsequent caliphs. The Abbasid revolution tapped "a complex and still-evolving welter of religious ideas and expectations". Its leaders abandoned their more radical followers once they attained power; they also disappointed their Alid supporters, and their success "set the stage for a more precise articulation of Sunni and Shi'i Islam" (the modern focus on Ali and his descendants "came to be formulated in the eight and ninth centuries").
There is "some controversy as to the point at which we can safely speak of a distinctive Islamic tradition" and "The Consolidation of Islam, 750-1000" begins with questions of identity: "the contribution of the urban middle classes to the parameters of Islamic civilization as they took shape in this period was decisive, if only because of the persistent centrality of the shari'a to Islamic identity."
Berkey describes Caliph al-Ma'mun's attempts to impose doctrine and the subsequent "caliphal abandonment of any pretensions to special religious authority". He covers the development of Shi'ism, including the separation of its Isma'ili and Twelver strands, and the formation of Sunni traditionalism ("what we now call Sunni Islam is, in a way, simply non-Shi'i Islam") around shared ideas about law, ijma or the doctrine of consensus, and the role and authority of the ulama: "the system of education was less one of the transmission of knowledge than it was one of the transmission of personal authority over the texts in which that knowledge was found, and also of socialization". And he surveys the ascetic and mystical strands of Islam.
"Medieval Islam, 1000-1500" begins with an apology for the use of "medieval" (mostly for convenience) and emphasizes the ongoing development and creativity within the period. Berkey includes a general outline of the political history of the Near East, but focuses on the tensions between religion and political authority, with the fragmentation of the latter and the presence of "alien" (mostly Turkish) military regimes. He outlines debates over a Sunni "revival" or "re-centering", which "sharpened opposition to innovations" and made jihad into an instrument for the enforcement of standards. And he describes common patterns in social and political organisation, the legal system, and the transmission of religious knowledge. (Madrasas, or schools offering instruction in jurisprudence, came as late as the 12th century and originated in Khurasan.) Sufism had an uneasy relationship with the ulama, and had connections with Shi'ism, but it became more structured as it was formalised into different turuq or "ways". Popular religion, often involving syncretic elements and the veneration of individuals as well as mystical ideas, posed a broader threat to the authority of the ulama.
Overall, The Formation of Islam is an excellent survey of the earlier history of Islam, managing to provide some structure to a hugely complex body of religious institutions and ideas as they evolved over nine hundred years. It does that without hiding the extent of that complexity, providing some details of individual people and events, glancing at sources, and touching on the major historiographical debates and alternative views. The result is aimed most obviously at those with some historical training, but should be accessible to lay readers already familiar with Islam and the region's history.