Question

Hello Rabbi!
I happened to leaf through the first book of the trilogy, most of which I already know from the notebooks, and I saw that there you try to argue that Kantian morality rests on God.
Although I may agree with your claim that morality must be based on the existence of God, it should be noted that this is not Kant’s claim.
Kantian morality, and in fact Kant’s whole system, cannot tolerate heteronomy, and therefore Kant takes care to remove it from the realm of morality decisively and explicitly in many places (see especially the Analytic of the Critique of Practical Reason), and not for nothing—you will not find a Kant scholar who agrees with your assertion that Kant assumes God as the basis of morality.
I am afraid that your mistake regarding Kant stems from a failure to distinguish between two different levels of Kantian morality.
The first level is the one discussed in the Groundwork of the Metaphysics of Morals, and in the Analytic of the first Critique, and it speaks about the autonomous morality that every private subject establishes, and about his duty to obey the categorical imperative.
The second level, discussed at length mainly in Religion within the Boundaries of Mere Reason, speaks about an additional duty that a person has: to establish a moral world, including society (“bringing the highest good on earth closer”). For the sake of this level, Kant requires the concept of God as the one who makes it possible for my moral effort, and that of society, to yield results in the real world and not be blocked by indifferent empirical reality.
The sentence you brought as supposed proof for your position, from Religion within the Boundaries of Mere Reason, does not mean, as you presented it, that God is the moral legislator for the private individual; rather, He is the legislator for a moral society—that is, He makes possible the existence of a society that will realize morality on earth.
If so, this sentence refers to the second stage of morality and not the first stage, which the Rabbi discusses in the book—and into that first stage, heaven forbid, Kant would introduce a heteronomous factor like God. According to Kant, the moral act is good not because God commanded it, but because autonomous reason established it.
It should also be noted, beyond this, that even the concept of God that Kant proves from the fact that human society must have the ability (“ought implies can”) to shape reality and turn it into something moral, is an almost completely empty concept, and all the “theological” additions that the believing person adds to it, such as “moral legislator” and “the greatest perfection,” are in Kant’s view purely subjective (and even this empty concept I am allowed to think of objectively only within the framework of “the primacy of practical reason over theoretical reason”).
In light of all this, it seems to me that it is clear that salvation and a proof of God’s existence will not come to us from Kant. Even if it is very possible that you are right, and morality must rest on God, it is a bit of a shame to force your own view onto Kant and present the readers of the book with a false picture of Kant’s view.