from James L. Perotti on responding to Kant.
Thus in Kant and The Problem of Metaphysics Heidegger described his task as that of laying the foundation of metaphysics and then noted that this task is in service of a later determination of the question about god (theology), world (cosmology), and man (psychology). In other words, Heidegger is anticipating that once he settles the laying of the foundation for metaphysics, he can then attend to building the disciplines which rest on that base.
Heidegger does succeed in laying the foundation for metaphysics, in a qualified way. In fact, he seems to have thrice succeeded: in Kant and The Problem of Metaphysics, in Being and Time, and in "What is Metaphysics?" Yet none of these "successes" satisfy him. In demonstrating the possibility of the comprehension of Being in Kant and The Problem of Metaphysics and Being and Time, he came to see the problem in a more original way, which undermined the success of the attempts. So he posed the question again in "What is Metaphysics?" and the Introduction to Metaphysics. These result in an ever better grasp of the problem; consequently he realizes the inappropriateness of his metaphysical direction and "overcomes" it. Because the question of god was to be addressed immediately after the foundational questions were determined, Heidegger never moved forward; the more orignal posing of the question of Being proved to have made a successful laying of the foundation less possible rather than more. Heidegger, despite his early anticipations, backed away from the question of god. Indeed, his way leads away from posing the question.
The Kantian task of laying the foundation for matephysics focuses one's attention on principles or grounds. Traditional scholastic metaphysics, as we have seen, used god as the ground for all things as creator and the ground of truth or knowledge, since the essences of created things are knowable (ontologically true) due to the manner of the divine creative action. Kant gave priority to the question of the ground of truth or knowledge but sought the ground within men. As a result, metaphysics could be grounded only on a demonstration that man is capable of knowning the Being (Seiendheit) of beings, capable of getting beyond an experience of the appearance of beings to their essences (their Being).
Kant failed to show how this could be possible. Heidegger succeeded by explaining the transcendental imagination as the human faculty which institutes transcendence, forms the horizon of objectivity within which the Being of beings is perceptible.