Kant on Theological Determinism
There are many ways in which agent-libertarian free will might be threatened, but one specific problem which Kant raises concerns the compatibility of such a conception of free will with the fact that humans are creatures, and thus created by an act of God. Kant raises this point explicitly in KpV:
But a difficulty still awaits freedom insofar as it is to be united with the mechanism of nature in a being that belongs to the sensible world, a difficulty which, even after all the foregoing has been agreed to, still threatens freedom with complete destruction. In this danger there is at the same time, however, a circumstance that offers hope of an outcome still favorable to maintaining freedom, namely that the same difficulty presses much more strongly (in fact, as we shall presently see, presses only) upon the system in which existence determinable in time and space is held to be the existence of things in themselves; hence it does not force us to give up our main supposition of the ideality of time as a mere form of sensible intuition and so as merely a way of representing things that is proper to the subject as belonging to the sensible world; and thus the difficulty only requires us to unite this supposition with the idea of freedom. That is to say: if it is granted us that the intelligible subject can still be free with respect to a given action, although as a subject also belonging to the sensible world, he is mechanically conditioned with respect to the same action, it nevertheless seems that, as soon as one admits that God as universal original being is the cause also of the existence of substance (a proposition that can never be given up without also giving up the concept of God as the being of all beings and with it his all-sufficiency, on which everything in theology depends), one must admit that a human being’s actions [5:101] have their determining ground in something altogether beyond his control, namely in the causality of a supreme being which is distinct from him and upon which his own existence and the entire determination of his causality absolutely depend. In fact, if a human being’s actions insofar as they belong to his determinations in time were not merely determinations of him as appearance but as a thing in itself, freedom could not be saved. A human being would be a marionette or an automaton, like Vaucanson’s, built and wound up by the supreme artist; self-consciousness would indeed make him a thinking automaton, but the consciousness of his own spontaneity, if taken for freedom, would be mere delusion inasmuch as it deserves to be called freedom only comparatively, because the proximate determining causes of its motion and a long series of their determining causes are indeed internal but the last and highest is found entirely in an alien hand. (KpV, Critical Elucidation of the Analytic of Pure Practical Reason, 5:100-1)