Epistemology is a part of philosophy that deals with knowledge and how we know our knowledge – or, as Francis Schaeffer puts in, “how we know, and how we know we know.” If you think about it, there are two ways to know things. You either absorb it through your surroundings by watching, listening, reading, etc. or you find it out on your own via cognitive reasoning skills. In logic these are referred to as induction and deduction.
Starting with the first, how do major modern religions and ideologies explain how we inductively gain knowledge? The best way to find out is to review their presuppositions.
Hinduism: The Hindu believes that all is one; thus, all reality is illusory. By necessity, this includes laws that govern nature as well as our brains and selves.
Naturalistic Atheism: A godless universe is random and meaningless. If our senses are the products of one cosmic accident after another followed by millions of years of genetic mistakes, there is no reason to believe that our senses and environment ought to be understandable or even real.
Agnosticism: The agnostic god is a god of the agnostic individual’s own making. In essence, since they believe in a god of their own mind, agnosticism works as a self-deifying worldview.
None of these religions can account for senses that are reliable or for a logical, orderly, coherent universe around us. They are too vague or impersonal or random, or they begin with the self.
To put it another way, if any of these religions were to be true, none of us could know anything at all.
Let us turn to deduction. Can we trust our own minds to reason properly enough to know things on our own? Is Descartes’ “Cogito, ergo sum” (I think, therefore I am) a rational starting point for personal knowledge – even knowledge of one’s own existence?
Again, consider the aforementioned religions, or even religions like Mormonism and Islam (whose holy books contradict themselves). Reason cannot begin with the self, nor can it begin with a vague, impersonal god, and especially not without any god at all. Descartes statement is circular; in order to think, he presumed his own existence. He then used this to prove his own existence. It could just as easily be said, “Sum, ergo cogito.” Using your own mind to fashion or imagine a rational god who made your mind rational would be just as arbitrary and circular; that is to say thinking would be irrational. But do not proponents claim their minds’ thoughts to reaching these religious conclusions? Every one of them is false based on their own standards. It is your classic case of reductio ad absurdum - they reduce themselves to absurdity.
How do you know that you know what you know? The reason is that the Bible is true. God has promised to uphold the universe orderly and consistently, and He has created us in His image to perceive and think reasonably. Certainly, in this sin-cursed world we do not always exercise the best judgment, but He has given us His Word for us to correct our thinking with.
“But wait!” one may object. Did not I use my senses to read and understand the Bible in the first place? Is not my position just as circular as any other religion? Yes, but – I will use the same inductive observation to learn about all the other religions as well. Only if the Bible is true, however, can this method be possible. Biblical Christianity is the only worldview that is self-attesting. It is the only religion that does not, so to speak, blow itself up.
I take the Bible on faith. When I start with that as my presupposition, everything else makes sense. “Come now, and let us reason together, saith the LORD.” (Is. 1:18)
Francis Schaeffer uses the phrase “how we know, and how we know we know” in many of his books, such as "Escape from Reason" and "The God Who is There."
One may wonder as to why I did not include Buddhism. But the Buddhist is either going to believe in an agnostic life-force sort of god, or in no god at all (as the current Dalai Lama does). As such he either falls into agnosticism or atheism, albeit a spiritualized one (if such a thing is possible).
The method of argument for Christianity I have been using is perhaps most thoroughly discussed in Dr. Greg L. Bahnsen’s book 'Presuppositional Apologetics: Stated and Defended' but is more easily and informally presented by Dr. Jason Lisle in his book 'The Ultimate Proof of Creation.' C. S. Lewis refers to it in the chapter titled “The Cardinal Problem with Naturalism” in his book 'Miracles,' and Dr. Cornelius Van Til uses it delightfully in his excellent evangelistic essay, “Why I Believe in God,” which he wrote in response to George Bernard Shaw’s essay “Why I am not a Christian.” In short if I don’t make any sense, or if one gets to be as fascinated with this subject as I am, these are some great sources to check out.