This study investigates the ontological foundations upon which philosophical knowledge of God is constructed, with particular emphasis on the constitutive role of ontology in Islamic philosophical theology. It focuses especially on realism and causality as fundamental principles for demonstrating the existence of God and for making sense of the ontological relationship between the Creator and creation. The study proceeds from the premise that an important part of the disagreement between theological and philosophical schools, as well as between Islamic and Western approaches to theology, stems from differences in their underlying ontological frameworks rather than from differences in their arguments and conclusions alone. It therefore seeks to clarify how philosophical conceptions of God, His attributes, and His actions are connected to prior ontological principles, and to show how differing interpretations of realism, causality, and the basis of the contingent's dependence on a cause give rise to divergent models of the relationship between God and the world. The study employs a comparative analytical method, examining fundamental ontological concepts, tracing their development within the Islamic theological and philosophical traditions, and comparing them with selected Western philosophical and theological approaches on the basis of relevant primary sources. It concludes that philosophical theology rests upon a system of ontological principles that provides the foundation for establishing God's existence and explaining His attributes and actions. It further argues that differing interpretations of realism and of the basis of causal dependence lead to fundamental differences in the way the world's relationship to God is understood. Finally, the study suggests that some of the major difficulties encountered in Western theology are rooted less in the findings of empirical science than in unsubstantiated metaphysical premises, and that the ontological framework developed in Islamic philosophy offers a more coherent and deeper approach for understanding the God-world relationship.