When non-theists get into discussions with religious people, the focus turns naturally to areas of difference; not just concerning beliefs per se, but underlying assumptions and presuppositions which serve as the foundation upon which one's beliefs rest. Most common in my experience is the distinction theists draw between "man's logic" versus "god's logic." The underlying assumption made by the theist in such a scenario is that man's logic "can only go so far," and the mind of god is far beyond our comprehension. From here, the theist either argues that this is why we need faith (usually argued in a sloppily Kantian way), or that the non-theist shouldn't rely on "man's wisdom alone."
It's been a while since I picked on my favorite Christian apologetics blog, Apologetics 315, so I'll share a quote posted recently attributed to Sir Robert Anderson that emphasizes this typical argument from theists:
[W]hile Divine truth is spiritual, and can only be spiritually discerned, human error is natural, and can be met on its own ground. We cannot “reason” men into the kingdom of God, but by reasoning we can expose errors which prejudice them against it.
Here the foundation is laid: one cannot be reasoned into the kingdom of god, which (to me) implies that one cannot reason oneself into the kingdom of god either. If this is the case, then god is (again reminiscent of Kantian epistemology) "beyond human reason."
This view merely begs the question: if god is "beyond human reason" and thus the only way to know god is via revelation (whether by the "internal witness of the Holy Spirit" or sensus divinitatis or some other form of specific divine communication), then how can we be certain of anything relating to the divine? If deity is beyond our ability to comprehend as calculus is to a five year old, then how can we know whether said "revelation" is indeed from god? How would a five year old even know what calculus is? It is, as many apologists argue god is to us, "beyond" the child's ability to reason and comprehend. The logical inference one should draw is, if god is beyond human logic and reason, then we should all be agnostics, rather than confident followers of a religion.
I have heard Christians respond to this concern by saying (as William Lane Craig says) that believers have the "internal testimony of the Holy Spirit." This is just a fancy way of saying "god talked to me" which doesn't really address the concern. How do you know it's the Holy Spirit testifying inside you? What does that even mean? I have also heard Christians say that god "opened my heart" or - as the Calvinists would say, "god made me able to comprehend and accept his truth." Again, this doesn't address the concern. God is, as the argument goes, beyond our reasoning capabilities. That implies that one cannot know whether the spirit of god is testifying, or whether god has transformed a person into a being capable of understanding. They may as well say "I felt god's presence" (and many do say that, in fact), for it holds just as much evidential value.
This justification for faith is yet another example of circular reasoning. How do they know god talked to them? Easy: they know god talked to them because god talked to them. They know god because god revealed himself to them, and they know he revealed himself to them because it was god who revealed himself to them. Duh.
In contrast, the underlying presupposition held by many who bear the "atheist" or "freethinker" label is that "human reason" is just called reason and it's pretty much all we've got. That which we can apprehend with our senses and process logically with our rational capabilities is all we have upon which to base one's beliefs. Why do we think this way? Because it's all we've seen. We haven't seen one shred of evidence for any kind of god that isn't based on an a priori assumption and/or desire. We also think this way because we have seen it work. Science is a living, growing, ever-evolving discipline of gaining and refining knowledge. We see its effects. Even those who deny or show disdain for science cannot escape her benefits. This is all the more ironic in our digital age when those ignorant and disdainful of science use that which only science has been able to produce (*cough*Facebook*cough*).
Everything we believe, know or think we know - whether correct or incorrect - we understand via our reason. How accurate our beliefs are is based on: 1) the quality of data/evidence available to us; and, 2) our ability to utilize reason. The latter requires sufficient training in logic and critical thinking. Even if god could reveal himself in a way that made sense to us, we would have to interpret that divine message with the only tool we have: reason. The pertinent question is whether we will learn to use our reason well.
Dead-Logic.com