Immanuel Kant has indeed initiated a new era in philosophy with his new ideas on epistemology and ethics with his works Critique of Pure Reason and Critique of Practical Reason. However, prior to these works, Kant underwent certain development in his philosophical thinking— initially as a rationalist, then eventually maturing to the philosopher that he is known for. In line with this development of Kant’s philosophical thought, the researcher’s particular interest is in his ideas on God and metaphysics. By reviewing Kant’s philosophical works especially in the Universal Natural History and the Theory of the Heavens , A New Elucidation of the First Principles of Metaphysical Cognition , and The Only Possible Argument in Support of a Demonstration of the Existence of God , we find out that his initial idea of God reflects his rationalist convictions as thought by his mentors from the LeibnizWolffian school. However, his critical project that attempted to put metaphysics on solid ground resulted in more doubt of its certainty. This affected especially his ideas of God, freedom, and immortality, which he believes can no longer be supported with the transcendent metaphysics he used to adhere to. This led Kant to concede that in the realm of pure reason, the idea of God cannot be satisfactorily justified. He eventually found a new ground where the idea of God can stand in his investigations on practical reason and morality.