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.

Fichte would identify all states of our minds with states of our body - perhaps not merely of our brain, but the whole body as an acting organism.

We can establish empirical criteria for free actions, and investigate human actions on the presupposition we are free.

Karl Marx left it to others to find the way beyond capitalism to a higher form of society. He saw his role as giving them as accurate a theory as he could of how capitalism works, which would also show them the reasons why it needs to be abolished and replaced by a freer and more human form of society.

Both Kant and Fichte thought of traditions of revealed religion as ways of symbolically (that is, with aesthetic emotional power) thinking about our moral condition. Both thought that religion would become more and not less powerful, emotionally and morally, if the claims of scriptures and religious teachings were taken symbolically rather than literally (whatever 'literally' might mean in the case of claims that are either nonsensical or outdated or historically unsupportable if taken as metaphysical or historical assertions).

When people think that moral problems can be solved by some simple strategy of calculation, that sets them up for ghastly overreaching. They think they can turn everything into a "science" the way mechanics was turned into a science in the seventeeth century. They want to turn everything over to technocrats and social engineers. They become shortsighted or simplistic about their ends, and they disastrously overestimate their ability to acquire the information they need to make the needed calculations.

I don't think Kant's approach to religion is any longer viable in its original form. But that does not mean it is simply wrong or that we cannot learn from it.