The problem is that many who reject Marx do not read him, or read him only by bringing prejudices to their reading that prevent them from understanding him.

As with many metaphysical and religious questions, Kant thinks they lie beyond our power to answer them. If you can't stand the frustration involved in accepting this, and insist on finding some more stable position which affords you peace of mind and intellectual self-complacency, then you will find Kant's position "problematic" in the sense that you can't bring yourself to accept it. You may try to kid yourself into accepting either some naturalistic deflationary answer to the problem or some dishonest supernaturalist answer.

Kantian ethical theory distinguishes three levels: First, that of a fundamental principle (the categorical imperative, formulated in three main ways in Kant's Groundwork); second, a set of duties, not deduced from but derived from this principle, by way of its interpretation or specification, its application to the general conditions of human life - which Kant does in the Doctrine of virtue, the second main part of the Metaphysics of Morals; and then finally an act of judgment, through which these duties are applied to particular cases.

It is absurd for anyone to think that Soviet "Marxism" is a correct application of the thought of Karl Marx. No doubt Soviet propaganda represented it this way. But who believes Soviet propaganda? It is remarkable (but maybe not so remarkable after all, when you consider their motives) that apologists for capitalism, who would not accept Soviet propaganda on any other point, are eager to agree with it on this point.

In fact people do not know enough about themselves and what is good for them to form a sufficiently definite conception of the general happiness (or whatever the end is) to establish definite rules for its pursuit.

We can't coherently deny, or even decline to affirm, that we are free.

Fichte is a necessary step to both Hegel and Marx.