The history of the prophets` life indicates that all of them have followed the same plans to invite people to the principles, without any difference. When a prophet revealed his invitation with some reasons, his addressed people were divided into believers and disbelievers. Belief is a matter of gradation in weakness and strength. Some grades of belief may be somehow mixed up with some kind of hidden idolatry. According to religious teachings, the criterion believers` existential grading and promoting lies in righteous and Divinely satisfied deeds; that is, belief and righteous action may be complement with regard to each other. The goodness and merit of one`s action as well as its agent`s righteousness are based on one`s grade of belief, and again, belief leads to the strength of belief. A Muslim always lives in the grades of Islam and faith. Comprehensively defined, disbelief is denial of Divinity, monotheism, prophecy, and a necessity of religion as such. This study has sought to analyze the grades of belief and disbelief and to examine the misleading definitions of belief and its relationship with action during the history of Islam and its effect in appearance of such groups as Kharijites, Murjieh, Mu'tazilites and Ash'arites, and attempts to discover the most important deviation in the views during the early days and centuries of Islam in the field.