This paper focuses on the use of religious arguments amongst Salafist groups who have conflicted with each other in actual and ideological levels in order to legitimize themselves in the Syrian civil war. In this paper, we will examine the main Salafist organizations Al Qaeda, Al Nusrah Front, ISIS and Ahrar al-Sham which are recognized as terrorist organizations by all countries. We will examine the other Salafist organizations in the context of their relations with these main Salafist groups. The importance of this study is through contribution to the examination of how legitimacy is created through these religious arguments in conflicts between the organizations that define themselves as Salafis during the Syrian civil war. In this paper, we will briefly explain the concepts of takfir (excommunication), bay’ah (oath of allegiance) and Khawarij and also show how these concepts are used in inter-group fights. This paper references data and information from the group’s own internet resources and other independent channels. Takfir allows ruling the infidels who deny widely accepted religious principles sent by God through revelation and prophet’s messages. It is seen that the religious arguments used by ISIS to eliminate the religious legitimacy of the other Salafist groups is takfir. ISIS uses takfir argument against other Salafist groups [who are seen as rivals themselves] to take them outside the religious legitimacy realm. By doing this, ISIS places all of its actions in a so-called religious legitimacy framework against these groups. The religious argument used by other Salafist organizations in the Syrian civil war, firstly Al- Qaeda and Al- Nusrah Front, to destroy the religious legitimacy of ISIS, is Khawarij. Emerged as the first political/religious differentation of Islamic history, Khawarij is known to be extreme use of takfir argument against all its opponents. Kharidjites were not considered as a legitimate religious sect due to the extreme and takfiri views throughout the history of Islam, these Salafist groups aim to destroy the religious legitimacy of ISIS by consubstantiating it with Kharidjites. At the end, we will focus on bay’ah (oath of allegiance) that emerged as a religious issue, especially in the context of Al- Qaeda, Al- Nusra Front and ISIS. The word “biat” as used in the Turkish language, is “bey’at” in Arabic language. As a socio-political act, the oath of allegiance means choosing and determining the head of state and the loyalty to it within the framework of Islamic law. ISIS claimed itself to be the only legitimate authority after the proclamation of caliphate. In this respect, Al-Qaeda removed ISIS from its ranks by declaring that ISIS has distorted its allegiance to Al-Qaeda. In the same time period, Al- Nusra Front has renewed its allegiance to Al- Qaeda, but next period ended the relationship with Al-Qaeda due to its evolution to Jabhat Fatah al-Sham from Hayat Tahrir al-Sham.