According to Plantinga’s reformed epistemology, as perceptual beliefs, religious beliefs are properly basic, and therefore need no additional justification. But as it has been said frequently, this idea may lead to relativism. In this paper, first, we argue that not only its relativistic aspect allows forreligious extremism, but also it could be used to justify that kind of extremism. Then, reciting some historical testimonies, including John Calvin, Khawarij, Ibn Taymiyye and Seyyed Qutb, we suggest that in principle, for many centuries extremists have derived a benefit from an idea similar to reformed epistemology to justify committing violence and other unacceptable behaviors.