It is a book written by Ahmed Amin, which deals with the idea of ​​the Mahdi, which is based on the principle of salvation, that it will appear at the end of time a man who frees people from the slavery of the material, and spreads justice and peace, after it has been crowded with darkness, oppression and aggression. It is a metaphysical idea, whose roots extend throughout history, exist in various cultures. If not all of it is called the Mahdi, or called his reformist call to Mahdia, nor did the divisions of the Jews and Christians not differ about the existence of this idea, as other religions believed in its existence, such as Zardashti, and Al -Barahiya. The book deals with the historical development of this idea in the Islamic system, in a geographical spot that extends from the country of Andalusia, to the countries of India, the writer reviews how the idea of ​​the Mahdi appears, and its continuation of the Fatimids, the Unified, Carmatians, Qadimiya, Babya and Sinussiyah, and the historical development of this idea is concluded by dealing with the Mahdi Revolution in Sudan late The nineteenth century.