Monotheism was adopted in the Arabic world as a unifying force: ‘Muhammad knew that monotheism was inimical to tribalism: a single deity who was the focus of all worship would integrate society as well as the individual' (ibid., p.175). However, Islam became more than that: another integral step towards the third stage. Several factors contributed to this.
The Koran was written in Arabic, therefore directly accessible to all literate people. There was no priesthood, sanctified intermediaries. Religion became more about the personal relationship with God and personal responsibility. While Christians at that time attempted to wipe out free-thinkers and non-conformists (as in the case of the Cathars), in the Islamic world they were not persecuted. Rationality, which is a characteristic of the third stage, became more prominent. Muslim scholars in the Middle Ages had a decisive role in moving from Platonic intuitionism to Aristotelian rationalism, which greatly contributed to the development of the Western world and the rise of science. They used paper and printing, and introduced algebra and Arabic numbers (that originated in India). In the 9th and 10th centuries, more scientific discoveries were achieved in the Abbasid empire than in any previous period. Islam was, at that time, a step forward in social organisation too, which is reflected, for example, in a greater egalitarianism and equality of genders (the right to inheritance and divorce). This all goes parallel with the trend of increasing the distance between God and humans: ‘In the Koran, al-Lah... is more impersonal than YHWH [Jehovah]. He lacks the pathos and passion of the biblical God' (ibid., p.167).
Yet, even Islam, from the start, was not immune to polytheistic influences (as exemplified by the so-called ‘Satanic verses'). The decline in Muslim society (due to numerous invasions and a geographical shift of trade routes and economic power) inevitably led to a retreat to the conventional stage, and as a consequence, the re-surfacing of polytheistic elements. Muhammad and the members of his family gained the status of deities and even imams ‘were revered as avatars of the divine, each one has been "proof" of god's presence on earth and, in some mysterious sense, made the divine incarnate in a human being' (ibid., p.190).