This study tries to show the ranges and limits of reason concerning religious knowledge in Islamic thought from the viewpoint of the Islamic philosopher Sheikh al-Ra’ees Abu Ali Ibn Sina (Avicenna). The study attempts to give an answer to a basic question: What are the ranges and limits of reason in religious knowledge? According to Avicenna’s viewpoint, is it absolute or limited? If it is limited, are its limits divinely or humanly? The study gives answers to these questions, following the descriptive approach through showing a set of concepts related to conceptual principles. We also have followed the analytical approach, based on the divisions of knowledge and reason according to Avicenna, in order to define the kind of reason he had meant, and to know the ranges and limits of reason in the field of religious knowledge. Besides, the spurious arguments raised against this approach have been discussed and answered according to Avicenna’s principles. It has become clear, through the article, that reason has a basic position in the exploration of religious knowledge that comes within the limits of reason.