[1] Drawing on archaeological evidence and contemporary documents in Arabic, Armenian, Coptic, Greek, Hebrew, Aramaic, Latin and Syriac, Crone and Cook depict an early Islam very different from the traditionally-accepted version derived from Muslim historical accounts.

Crone and Cook contend that an alliance of Arabs and Jews sought to reclaim the Promised Land from the Byzantine Empire, that the Qur'an consists of 8th-century edits of various Judeo-Christian and other Middle-Eastern sources, and that Muhammad was the herald of Umar "the redeemer", a Judaic messiah.

This borrowed key concepts from the Jewish breakaway sect of Samaritanism: "the idea of a scripture limited to the Pentateuch, a prophet like Moses (Muhammad), a holy book revealed like the Torah (the Quran), a sacred city (Mecca) with a nearby mountain (Jabal an-Nour) and shrine (the Kaaba) of an appropriate patriarch (Abraham), plus a caliphate modeled on an Aaronid priesthood.

Muhammad would have claimed such descent for Arabs to give them a birthright to the Holy Land and to prepend a monotheist genealogy compatible with Judaism to their pagan ancestral practice (such as sacrifice and circumcision).

8-9) The authors, interpreting 7th century Syriac, Armenian and Hebrew sources, put forward the hypothesis that Muhammad was alive during the conquest of Palestine (about two years longer than traditionally believed; the caliphate of Abu Bakr was hence a later invention).

He led Jews and Hagarenes (Arabs) united under a faith loosely described as Judeo-Hagarism, as a prophet preaching the coming of a Judaic messiah who would redeem the Promised Land from the Christian Byzantines.

In order to break with Jewish messianism, they recognised Jesus as messiah (though rejecting his crucifixion), which also served to soften the initially hostile attitude towards a growing numbers of Christian subjects.

The Quran was expeditiously collected from earlier disparate Hagarene writings, possibly heavily edited into its complete form by al-Hajjaj (that is, in the last decade of the 7th century rather than the middle, under Uthman, as traditionally believed; see Origin according to academic historians).

As power was transferred from Syria to Iraq, Islam incorporated the rabbinical culture of Babylonian Judaism: religious law practised by a learned laity and based on oral traditions.

In the second half of the eighth century, the early Muʿtazila, simultaneously with Karaite Judaism, rejected all oral traditions, leading to a failed attempt to base law on Greek rationalism.

Part I of the book ends by considering the peculiar state in which the Hagarenes found themselves: their own success pushed them away from the sanctuaries of Jerusalem and Mecca to Babylonia, as finalised by the Abbasid Revolution; Umar had already lived and there was no lost land or freedom to hope for.

The remainder of the book, Parts II and III, discuss later developments and the larger context in which Islam originated: the Late Antique Near East, and relate it to theoretical themes of cultural history.

"[9] Crone and Cook's work was part of revisionist history[13] arising from several scholars associated with the University of London's School of Oriental and African Studies (SOAS), beginning in the 1970s.

Hagarism was acknowledged as raising some interesting questions and being a fresh approach in its reconstruction of early Islamic history, but it was described by Josef van Ess as an experiment.

... As a book making a specific argument, Hagarism was ultimately a failure, but in its stimulus of further research, writing, debate, and especially by challenging Islamicists to look beyond the confines of Arabic sources to the rich literatures of the Middle East that existed before, during, and after the rise of Islam, Hagarism was one of those rare books that changed a field.Stephen Humphreys, professor at UCSB, wrote in his analytic review of the historiography of early Islam:[3] Unsurprisingly, the Crone-Cook interpretation has failed to win general acceptance among Western Orientalists, let alone Muslim scholars.

But, he says that "most, if not all, [of the sources] have been or can be challenged on suspicion of inauthenticity" and that "the material is upon occasion misleadingly represented ... My reservations here, and elsewhere in this first part of the book, turn upon what I take to be the authors' methodological assumptions, of which the principal must be that a vocabulary of motives can be freely extrapolated from a discrete collection of literary stereotypes composed by alien and mostly hostile observers, and thereupon employed to describe, even interpret, not merely the overt behaviour but also intellectual and spiritual development of the helpless and mostly innocent actors.

Brown wrote in Apocalypse and/or Metamorphosis (1991) that Hagarism, "illustrates in an ominous way the politics of Orientalism", and citing Grabar's review, added that, "The Western tradition of urbane condescension has degenerated into aggressive, unscrupulous even, calumny".

"[21] Fred Donner, reviewing Hagarism in 2006, viewed the book as a "wake-up call": despite initial repudiation, it set a milestone by pointing out that scholars need to "consider a much more varied body of source material than most were used to using, or trained to use."