SUBJECT & OBJECTIVES: Since the early centuries of Islam, there have been numerous discussions on the subjects of disbelief and faith, as well as examples of both in Islamic society, which refer to believers and disbelievers. Meanwhile, the authors of sectarian books like “MILAL wa NIHAL” have tackled this matter from a sectarian perspective, frequently attempting to differentiate the Heretical sects from the Saved ones. Based on this, this research aims to examine the approach of Islamic sect-historiographers on faith and disbelief and happiness and misery.METHOD & FINDING: Based on the works of Islamic Islamic sect-historiographers who have benefited from a descriptive-analytic approach, this study covers the first five centuries of Islamic history. Research indicates that the topic of faith and disbelief in the writings of Muslim sectarians is influenced by the sect to which they belonged, to the extent that the regulation and prioritization of this issue is evident in their works.CONCLUSION: The findings of the research demonstrate that Muslim sectarians occasionally attempt to disparage rival sects in their writings by employing improper language, blaming them for not believing in a Hereafter, or even accusing followers of other sects of being deviant Muslims.