This paper tries to reveal the religious ideologies in the discourse of ISIS's Rumiyah. The main objective of this paper is to present a linguistic investigation that might help in deciphering the underpinning ideologies beyond the mere linguistic form of Rumiyah's discourse. Two research questions are addressed in this paper: first, what are the main religious ideologies around which ISIS's discourse in Rumiyah revolve? Second, how can these religious ideologies be deciphered by linguistic tools? To achieve its purpose, the paper draws on Michael Halliday's (1985, 1994) systemic-functional approach, specifically his three semantic meta-functions: ideational, interpersonal and textual; and the semiotic functions represented in the three variables of register: field, tenor and mode. To decipher religious ideologies in the selected data at the semantic and semiotic levels, three main lexico-grammatical resources are employed: transitivity, mood and theme. Two main findings have been highlighted in this paper: First, ISIS's discourse in Rumiyah is semantically and semiotically structured around two main ideologies: jihad and jama'ah, which, in turn, tends to reformulate the social, political and religious attitudes of its readers. Second, by establishing a compatible point of contact between ideological discourse analysis and systemic-functional analysis, the paper shows that ISIS's Rumiyah is linguistically structured around one extremist opposition-oriented discourse that displays two irreconcilable ideologies: a positive triumphant selfness versus a negative beleaguered otherness