This paper studies fundamentalism, from the perspective of the tension between communicability and incommunicability, perceived in the religious dynamics in general. After a brief historical background on the subject, it focuses on the religious side of fundamentalism. Next, fundamentalism is approached in the context of its specific contradiction: the incommunicability of dogmatism, based on literalism, the incapacity for dialogue and extremist political actions, including terrorism, in permanent tension with the communicational need for visibility and recognition. Therefore, there is a specific communicational condition within the incommunicability of the processes of dogmatization and dialogical closure inscribed in fundamentalist phenomena. We call this a communicative propensity to promote incommunicability. Finally, religious fundamentalism is postulated as a proselytizing of legitimation, tensioning the poles between dogma and bond, by adopting, contradictorily, the radicalization of both, and testing the limits of tolerance in modernity