The typical three-stage bare-bones summary of classical modern philosophy is: Descartes' Rationalism versus Hume's Empiricism versus Kant's Idealism. All three are theories in epistemology. Most of the philosophy in that astonishingly rich two-hundred-year period between the publication of Descartes' Discourse on Method in 1637 and the death of Hegel in 1831, the period of classical modern philosophy, was concerned with epistemology. "Epistemology" means "theory of knowledge". (What is knowledge? How do we know? How does it work? How should it work?) It is probably the trickiest and most purely theoretical division of philosophy. Yet it is foundational, for any position you take in epistemology will always have consequences for, and make a great deal of difference to, all the rest of your philosophy: your metaphysics, cosmology, philosophical theology, anthropology, ethics, and political philosophy.This is interesting to me. It is precisely the weak point of protestantism. They abandoned the balance between faith and reason that formed the foundation for all human knowledge. Luther and Calvin used reason to destroy whatever parts of faith they desired. Others came along and destroyed what they had built. By the time Descartes came along the need for a new foundation was clear.
I do agree that epistemology is "probably the trickiest and most purely theoretical division of philosophy". That is because we need to question our own minds. To question the way we ask questions. To take the things we have a psychological certainty about and ask whether that certainty is based on something that might be false.
We do this for other people. We can analyze a Jehovah's Witness or an atheist and we can figure out how he became so sure about things that are not true.But doing it for ourselves is way harder. Many of these beliefs are at the very core of our self identity. There is a huge risk in seriously asking them.
At the root of this is a lack of faith. We don't want to put our mist cherished beliefs up to scrutiny because we are afraid of what we will find. I know as a protestant I would have said that about Catholics but not about myself. I thought I had a very rational faith and I didn't think there were any question I was avoiding. I thought most Catholics were afraid to learn their faith because if they did they would become protestants. Catholics who had become protestants confirmed this for me. Their faith just didn't stand up if and when they dared to scrutinize it.
Then I found those who did the opposite. Those who asked questions about their protestant faith that I never asked. Amazingly enough they ended up Catholic. It turns out I was not better than those Catholics or Jehovah's Witnesses. I found it very uncomfortable when my faith tradition was being questioned. Again it was a lack of faith. I didn't trust the gospel of Christ to stand up in some form. I was afraid that if I applied rational skepticism to them all they would all be destroyed. I didn't call it fear. I called it faith. Having enough faith not to ask questions. Not about everything. Just about a few things.
But it is that personal connection we have to our own minds that makes epistemology so hard. I think something is clear but maybe not everyone does. Is it really clear then? I find that is the biggest barrier when talking with protestants. The natural human tendency to see their own mind as the center point. They believe that sin darkens their intellect but they don't quite grasp the full implications of what that means. That Luther's idea of using "scripture and plain reason" to destroy long settled doctrine is a recipe for disaster.