The extent and distribution of studies conducted in the field of religious identity is such that there is no coherent image and vision of the data and scientific findings in the area of measurement and analysis of factors affecting religious identity. Therefore, a systematic review of the conducted scientific research, evaluation of the methodology and findings of these investigations as well as the way religious identity has been assessed in them can be an effective step to draw a comprehensive picture of the conducted studies. Accordingly, while classifying and integrating the conducted studies in the field of religious identity, the present study answers the question of how religious identity has been defined and measured in previous research. "Meta-synthesis" method was applied to answer this question in a way that after integrating the studies, analyses were presented. The results indicate that there has been little organized effort to study religious identity with a local and comprehensive approach. This negligence is visible both theoretically and methodologically from the part of humanities and social sciences researchers. On the other side, many studies conducted in this field do not have a similar perception of religious identity in a way that a type of reductionism is seen in the definition and measurement of this concept by researchers.