The rationality within the network of religious beliefs in theological systems demonstrates a hierarchical relationship among them, where each belief possesses rationality corresponding to its existential position within the belief structure. Accordingly, three levels of religious beliefs can be identified: fundamental, intermediate, and peripheral beliefs. In this hierarchy, fundamental beliefs possess maximum rationality, followed by intermediate beliefs, while peripheral beliefs exhibit the least rationality compared to higher-level beliefs. An examination of these belief levels reveals two shared fundamental beliefs in Islamic and Christian theology: faith in one God and belief in the afterlife. The common intermediate beliefs of both traditions primarily concern divine attributes, the relationship between humans and God, the doctrine of creation, and similar themes. Additionally, distinct intermediate beliefs can be observed in Christian theology (e.g., Trinity, Incarnation, Atonement, Original Sin) and Islamic theology (e.g., divine attributes and essence, temporality/eternity of the Quran, Imamate and Caliphate). Finally, peripheral beliefs in both traditions relate to the sanctity of places, times, events, objects, individuals, and circumstances. This study demonstrates that while all religious beliefs are subject to epistemic and non-epistemic factors and undergo historical expansion/contraction, the fundamental beliefs of these two traditions exhibit greater stability, universality, and alignment with human nature, making them less susceptible to such influences and thus more rational. Islamic and Christian intermediate beliefs are largely shaped by these factors and reflect diverse interpretations of sacred texts, whereas peripheral beliefs depend on multiple variables and are more prone to change. The key finding is that the rationality of belief networks in Islamic and Christian theology diminishes from fundamental to peripheral levels and is constantly influenced by various expansive/contractive factors. Consequently, engaging with religious rationality requires careful consideration of how these belief networks develop and transform under different influences.