'Belief' was historically the first and the most important of all the theological concepts in Islam, and it raised in the first few centuries of Islamic culture a number of problems of real significance. Some of them were literally problems of life and death to the growing Muslim community, as they are still today. By methodically analyzing these problems and the major concepts underlying them, this study throws light upon one of the most interesting phases of the history of Islamic thought. 'The danger of the free practice of takfar', 'the relation between aman and islam', 'belief and knowledge', 'belief and work' are some of the key concepts explored by the author in depth and analyzed in the light of earlier studies, such as the works of Imam Ghazali and Imam Ibn Taymiyyah. This work by Dr. Toshihiko Izutsu (1914-1993), one of the eastern world's greatest modern Islamic scholars, is one of three in a series of works by the late scholar re-issued by us.