Van Til is a master at exegeting unbelief. This is helpful for apologetics. If we do not understand the unbeliever in a biblical way, inevitably our approach to defending the faith will be unbiblical.
Therefore, it is very important to know how the unbeliever thinks about reality (ontology) and knowledge (epistemology). In terms of the former, consider this quote from An Introduction to Systematic Theology:
They have thought of God either as enveloped in the universe or as removed from it so far that it really existed independently of him. (p. 142)
Van Til concisely underscores the two modes of ontology found in unbelief. I say here two, but really they are one. On the one hand, the unbeliever begins with a god that is “enveloped” in the universe. Obvious examples of this mode is found in pantheism, Hegelianism, process theologies, etc. Less obvious, however, is analogia entis forms of thinking. Van Til typically (though by no means exclusively) finds this in Roman Catholicism, especially in the metaphysics of Aquinas. For Van Til Aquinas believes in a chain of being between God and the rest of creation. In this system “being” is an abstraction. That means being takes on a life of its own, sort of speak, which “envelops” both the Creator and the creature. Being is something that is absolute, and as such is shared by both God and man. Of course, Van Til is not saying Aquinas is an “unbeliever,” rather the charge is that he adopts an unbelieving mode of ontology into his thinking. This is the mode of thinking which Van Til calls “rationalism.”
On the other hand, the unbeliever so separates God from creation, creation ends up having an independent existence all its own. Obvious examples of this mode is found in deism, Kantianism, etc. Less obvious, it would also be found in Karl Barth. We’ll unpack that idea in a future post. For now, suffice it to say that this mode of thinking is what Van Til calls “irrationalism.”
But here’s the zinger: the second mode, no less than the first, ultimately envelops God within the created order. This is how. If God is so removed from the creation, i.e., “wholly other,” and he has not revealed himself directly to us, then he is ultimately unknowable (this is what Van Til calls a “non-Christian view of mystery”). And if God is ultimately unknowable, then we must exercise our “would-be” independence and autonomy to interpret the world around us quite apart from God. This move turns our irrationalism into rationalism, and it (in effect) exalts “would-be autonomous man” to become god-like. Deism turns into pantheism! The unbeliever inevitably looks to something in creation to be the final arbiter of what is true. This final arbiter becomes, for the unbeliever, not only the decoder ring (my metaphor, not Van Til’s) for understanding reality, but also the judge and jury over God himself. To use an expression of C. S. Lewis, on this approach God is placed “in the dock.” He is on trial, and the creature will judge the Creator. So, Van Til’s observation:
Whether in science, in philosophy or in religion, the non-Christian always seeks for a daysman betwixt or above God and himself, as the final court of appeal. (Common Grace and the Gospel, 11)
I love this quote for many reasons. But particularly for its use of “daysman.” It comes from the King James translation of Job 9:33, “Neither is there any daysman betwixt us, that might lay his hand upon us both.” For comparison, here is the ESV translation: “There is no arbiter between us, who might lay his hand on us both.” I would urge the reader to go and read that verse in context, its quite profound. I won’t exegete it here, but will only say that Van Til’s allusion to it is brilliant. God is God and he will not be put on trial by his creatures. And that is the essence of unbelief. Unbelief is the creature putting the Creator on trial by invoking a “daysman” from within the very creation of the Creator to judge Him.
At the end of the day, this is where all unbelief ends. It all ends with a violation of the Creator-creature distinction. It is all—whether pantheism or deism—an attempt to exalt the creature above the Creator. It is an attempt to make God not God. It is an act of cosmic treason, which is idolatry.
 For a helpful “exegesis” of unbelief in the tradition of Van Til, see the lectures on the anatomy of unbelief by K. Scott Oliphint found here.
 I am aware of recent protestations against Van Til’s understanding and critique of Aquinas. For an answer to those protests, an answer I find absolutely compelling, see K. Scott Oliphint’s lectures on Aquinas here.