[In the following excerpt, Gibb mentions Ibn Hazm's Kitāb al-fasl fi al-milal wa al-ahwā' wa al-nihal, calling it “the first systematic and critical work on the religions of mankind.”]

The chief figure in the prose literature of the eleventh century is Ibn Hazm of Cordova, the grandson of a Spanish convert. In his early years he was pre-eminently a poet, but, belonging to the narrowest school of Islamic theology, his activities were diverted to bitter attacks on his theological opponents; the sharpness of his tongue, which became proverbially linked with the sword of the tyrant al-Hajjāj, eventually forced him to give up political life and brought about his practical excommunication. Of his immense theological and historical activities little has come down to us beyond his valuable and original work on Comparative Religion ([Kitāb al-fasl fi al-milal wa al-ahwā' wa al-nihal] The Book of Religious and Philosophical Sects). Strange though it may appear that it is in Arabic literature that we find the first works on this subject, the reasons for it are not far to seek. The tolerance of the Arab conquerors had left in their midst large communities holding most varied religious opinions, Jews, Christians, Zoroastrians, and even semi-pagans. The contrast between these beliefs and their own attracted the attention of Muslim scholars at an early date and led first to a large controversial literature (as a specimen of which may be cited the Book of Religion and Empire written about 855 by ‘Alī b. Rabban aṭ-Tabarī, himself a convert from Christianity) and later on to a more scientific curiosity about them. There were also administrative problems connected with the special taxation and jurisdiction of the non-Muslims, which made it necessary for officials to have some knowledge of their creeds and practices. The rise of different canonical schools within the Islamic community itself led to the writing of works on comparative doctrine, generally with a controversial purpose, such as the Distinction between the Sects of Abū Manṣūr al-Baghdādī (d. 1037), which does not deal with any group beyond those which claimed to be included in Islam. It was reserved for Ibn Hazm, however, to write the first systematic and critical work on the religions of mankind, including their various sects and schools. His book opens with a theologico-philosophical classification of religions according to their beliefs on the origin of the world and the vocation of prophets; Christianity, for instance, comes under the category of creeds which assert that the world was created in time and had more than one creator, and which reject certain of the prophets (i.e. Muḥammad and the Arabian prophets). In each section he details the arguments advanced in support of these beliefs and follows these with a refutation point by point. A large section of the book is devoted to a trenchant analysis, quoting chapter and verse, of the inconsistencies and, to the Muslim mind, absurdities contained in the Old and New Testaments. The various Muslim sects and philosophical schools are then discussed, and the work ends with an exposition of Ibn Hazm's own philosophical and theological views. It is clear that the purpose and style of the whole book is controversial, but it led to more judicial works on the same subject.