This research aims to explain the fundamental role of ontology in the formation of political philosophy and compare it in the three traditions of Christianity, Islam and modernity. The main argument of the article is that every political philosophy, before being a set of political propositions, is based on an ontological view of the three basic components of "God", "nature" and "man". Accordingly, the article first presents a three-fold model for the analysis of ontology and then shows how the difference in understanding the existence of God, the nature of nature and the nature of man leads to fundamental differences in political concepts such as freedom, justice, legitimacy, sovereignty and the relationship between religion and politics. In the pre-modern Christian tradition, ontology is based on the axis of original sin and existential hierarchy, and this has led to a politics based on obedience, abstinence and separation between the "city of God" and the "earthly city".In the Islamic tradition, monotheistic ontology, relying on the concept of "human caliphate" and divine continuity in existence, has laid the foundation for a political philosophy that emphasizes justice, moral responsibility, and harmony between nature, society, and the divine. In contrast, modernity, with its anthropocentrism and scientific naturalism, has removed God from the political process and redefined politics based on self-centered rationality, individualism, and social contract. According to the research findings, the fundamental differences between Islamic political thought and modern political philosophy lie not simply at the level of ethics or jurisprudence, but also at the level of ontology. As a result, political philosophy, without paying attention to existential and metaphysical foundations, loses the possibility of explaining its key concepts, and understanding politics inevitably depends on understanding existence