This is the basic argument presented by From the History of Atheism in Islam by the renowned Egyptian thinker Abdel-Rahman Badawi. Published in Arabic in 1945, the book was reprinted only once in 1993. It discusses the work of the Islamic philosopher-scientists of the medieval period and the way they upheld reason, freedom of thought and humanist values, while questioning and often refuting some basic Islamic tenets.
Although many of those thinkers, according to Badawi, did not attempt to disprove the existence of God, they lashed out against the notion of prophethood and argued against the privileged position occupied by the Prophet Muhammad and his followers.
Most prominent among those scholars was Abu Bakr al-Razi (865-925 CE) who believed in the supreme importance of reason. He argued that the mind had an innate capacity to distinguish between good and evil, and between what was useful and what was harmful. According to him, the mind did not need any guidance from outside it, and for this reason the presence of prophets was redundant and superfluous.
Al-Razi directed his most vehement attack against the holy books in general, including the Qur’an, because he saw them as illogical and self-contradictory. He also believed that all human beings were equal in their intellectual capacities as they were in all other things. It made no sense therefore that God should single out one individual from among them in order to reveal to him his divine wisdom and assign him the task of guiding other human beings. Furthermore, he found that prophets’ pronouncements and stories often contradicted those of other prophets. If their source was divine revelation as is claimed, their views would have been identical. The idea of a divinely-appointed mediator was therefore a myth.