Spencer's important biography of Muhammad deserves a wide audience. Its central argument can be summarized as follows: Islam requires its adherents to obey and imitate Muhammad, holding him up as "the perfect person." Indeed, over two dozen verses in the Qur'an command Muslims to do this. Yet the earliest and most reliable sources we have on Muhammad (all of them written by Muslims: the Qur'an, the early Muslim biographies of Ibn Ishaq and Ibn Sa'd, and the collections of prophetic traditions assembled by Bukhari and Muslim) testify that Muhammad engaged in, commanded, and condoned actions that any civilized person today would condemn as gravely immoral.
He commanded the murder of those who mocked him, the execution and enslavement of hundreds of prisoners of war, the killing of apostates, the forcible conversion of pagan Arabs, the military subjugation and humiliation of Jews and Christians, and the torture of prisoners, and permitted sexual relations with female slaves and war captives, to mention just a few examples. He also manifested angry contempt for anyone who questioned his claim to be God's messenger, especially the hapless Jews of Medina, who made the fatal mistake of telling him the evident truth that their scriptures do not foretell his coming as the final prophet.
Spencer concludes by noting that contemporary Muslim extremists and militants model their behavior on Muhammad. Their violence and intolerance, far from being alien to an inherently peaceful religion, is directly rooted in Muhammad's example. His powerful central argument demonstrates that many of the problems in the Muslim world today can be traced directly to the regrettable precedents set by the founder of the faith.
To the common argument in defense of Muhammad (e.g. by Karen Armstrong) that one must not judge a seventh-century Arab chieftain by contemporary ethical standards, Spencer replies: Perhaps so, but Muslims must choose. Either Muhammad embodied the morally deficient standards of a barbarous age, or he is the perfect person and hence the standard by which present practices are to be evaluated. Muslims cannot have it both ways but must choose. However, each option poses a problem. Muslims who forsake Muhammad will be condemned for abandoning 1,400 years of Islamic tradition; those who insist on the perfection of the prophet implicitly condone barbarism.
Spencer's book does suffer from some flaws, starting with the publisher's sensationalistic title that belies the seriousness of the book. In some chapters, endnote numbering does not match the sources cited, making it difficult for the reader to follow up on references. Better editing would have detected factual errors: pages xii and 154 incorrectly date the Battle of Tabuk to 631 while elsewhere the author correctly dates it to October 630. During the Battle of the Ditch, Muhammad proposed buying off the Ghatafan, not the Quraysh. It appears that Muhammad condoned the intentional killing of women and children in war when, in fact, he condemned this; the hadith quoted seems only to condone collateral or unintentional killing on night raids. The same applies to the assertion that Muhammad condoned the mutilation of enemy dead on the battlefield whereas he prohibited such mutilation, as Spencer's own source, Ibn Ishaq, makes clear. Qur'an 8:65 is quoted without noting that the verse immediately following abrogates it. The opening verses of Qur'an 9 were revealed and promulgated during the hajj in March-April 631, not during Muhammad's farewell pilgrimage in 632.