It is the case that if Kant’s Prolegomena claims that we cannot know anything of God (or other supernatural things for that matter) through rational thought, then, if it is right we should not be doing any rational theology at all. This is because if we cannot know anything of theology through rational thinking alone then rational theology is not a study worth pursuing since there is nothing that can be known about it or come from it.

And Kant’s Prolegomena does seem to claim this (that we cannot know anything of God or other supernatural things.) Kant clearly writes “The deistic concept is quite a pure concept of reason, but represents only a thing containing all realities without being able to determine any one of them.” (Prolegomena to any Future Metaphysics, Immanuel Kant.) This quote clearly conveys that Kant does not believe and claims that we can only know that God exists, but nothing about God. Later in this same work, he makes similar arguments about knowing anything about the afterlife, the soul, and anything of that matter.

Therefore, it follows that if Kant is right (in the Prolegomena) about these things, we should not be doing rational theology at all. If we cannot know anything other than the existence of something from rationality, then we should not attempt to understand it through rationality.

Of course, an objection to this could be that it concludes that we should not be doing theology at all, which is clearly wrong. This is not what I was trying to say, rather I am simply saying that if Kant is right in the Prolegomena (not that he is right) then we should not be partaking in rational theology. That is, claims of a priori knowledge of God and other supernatural things (other than that of just His or their existence) cannot be a thing, according to Kant, because we cannot know anything of Him or other supernatural things. However, this is not to say that we should not be employing experiential a posteriori) knowledge to give us clues to how God or other supernatural things function.