This paper focuses on the comparative analysis of the notion of Grace in Gottfried Wilhelm Leibniz's Principles of Nature and Grace and in Immanuel Kant's Religion within the boundaries of Reason Alone. The primary objective is to show how this crucial theological concept is treated divergently by the two thinkers, reflecting the epistemological and metaphysical differences between Leibnizian systematic and metaphysical rationalism and Kantian transcendental critical philosophy. In Leibniz, that of grace is not an arbitrary intervention, but a fully integrated element in his philosophical system of pre-established harmony. It represents the rational and intrinsic action of God in the "Kingdom of Grace," a domain that elevates rational souls in a manner perfectly in accordance with the divine plan for the creation of the best of all possible worlds. Immanuel Kant brings the traditional notion of grace to a severe and radical critical reinterpretation, in line with the founding principles of his transcendental philosophy, while still maintaining the desire for a close union and reconciliation between the moral and scientific worlds. We argue here starting with an analysis on the transcendental reworking of the notion of teleology that Kant operates in his third critique, The Critique of Judgmenet, with respect to the Leibnizian understanding of the question. An attempt is then made to use the proposed analysis as an important case study to illustrate the fundamental shift toward that Kantian redefinition of the relations between reason, faith and morality that would open the season of German idealism.