Gordon Clark is oftentimes accused, particularly by his Van Tillian opponents, of being "rationalistic" in his approach to the Christian doctrine of knowledge, and in his apologetics. By "rationalist", they are not referring to the epistemological approach contrary to that of empiricists, but rather, using the term in a pejorative sense such that the one being accused of elevating reason to such a degree that it is given authority that it ought not have. The accusation usually brought against Clark and his disciples has to do with the elevation of logical thought to autonomous reason.
For Clark, Scripture teaches that God is logical in his thought, and since he is logical in his thought, it follows that he is logical in the revelation of his thought. He reveals His logical thought to His creatures, and since His revelation is logical, it follows that He would have created creatures whose thought is logical in a way that mirrors God's own thought. Otherwise, these creatures would not be able to understand God or His revelation.
So it is important to keep in mind that in light of Clark's own self-understanding, he is a pure presuppositionalist. He does not believe that autonomous reason or logic can disprove non-Christian worldviews. Rather, he believes that logic has divine sanction, and when he points out logical inconsistencies in the worldviews of unbelievers, he is demonstrating that they are suppressing the truth in unrighteousness by means of their God-given logical (though inconsistently so) thought.
In reality, this apologetic strategy, and the presuppositions undergirding it, is really not that different in spirit from the transcendental argumentation as employed by Van Tillians. Van Tillians argue that only the Christian worldview provides the preconditions of intelligibility for knowledge. Any other worldview which employs the rational thought of Christianity apart from these preconditions is simply inconsistent. Indeed, a similar criticism of logical consistency is used by Van Tillians, just as it is used by Clarkians.
So I believe that in this respect, Van Tillians and Clarkians are closer to one another than many Van Tillians realize. That is not to deny that there are real, and ultimately irreconcilable, differences in epistemology between the two. There are. But Van Tillians need to understand that Clarkians see their own apologetic technique as purely and faithfully presuppositional. They believe in the use of logic not because they elevate autonomous reason inordinately, but because they believe that God is logical, that our thoughts are logical because we are so constituted by God so that we can understand God's logical thought, and that this logic therefore has a divine sanction.
Here, I think, is the main reason Van Tillians believe that Clarkians are rationalistic: Van Tillians, in their antithetical apologetics, tend to operate with a criteria of rationality which I will refer to here as primordial rationality. That is, for Van Tillians "rationality" refers broadly speaking to a worldview's criteria of the right way to think. "Logic" and "reason" are only particular species of right thinking, and one's worldview dictates precisely what counts as logic or reason. So this primordial rationality, which refers to the "right way" of thinking, broadly speaking, includes logic and reason as only particular species of right thinking. For Clarkians, however, the Bible teaches that thinking is right if and only if it is logical.
Keep in mind that for a Van Tillian, it is one's worldview which dictates standards of rationality, intelligibility, and so on, and so one's worldview will dictate what sorts of things will count as "correct" thought. The Bible teaches the Trinity and the hypostatic union, and Van Tillians believe that these things do not conform to propositional logic or ordinary human reason. We therefore cannot apply such logical standards to such truths. This is based on an exegetical difference between them and Clarkians: They do not believe that the Bible teaches that logic is the essence of correct thinking. They simply believe that the Bible is true, and we are to believe its truths. Some of its truths may conform to the dictates of propositional logic, but others might not. Clarkians believe on a purely exegetical basis that the Bible teaches that the essence of correct thought is logical thought.
Clark does admit that truths like that of the Trinity or the resurrection cannot be deduced independently of the Bible, and so he clearly denies that autonomous human reason can be used to deduce the truths of the Bible. Indeed, according to his scripturalism, the Bible is our epistemological axiom, and apart from it we cannot know anything at all. So he is keen to remain a consistent presuppositionalist. There is indeed such a thing as being inordinately rationalistic. Anselm, who believed that we could deduce the doctrine of the Trinity autonomously, is indeed an example of this sort of rationalism. And indeed, such rationalism is absurd. But Clark cannot be accused of this sort of rationalism.
I think that many Van Tillians look askance at Clarkian accusations of illogical thought in their dialogue with unbelievers, and wonder why Clarkians don't just reject unbelieving thought as "apostate" rather than, strictly speaking, as illogical. To a Van Tillian who does not believe that propositional logic has the sanction of the Bible, this does indeed come across as inordinately rationalistic.
Keep in mind also that a Van Tillian may object that if we cannot autonomously or independently deduce doctrines like the Trinity or the resurrection, it follows that logic is not the essence of right thinking. But for a Clarkian, the point is simply that all truth must be in conformity with such logic, not that we must be able to deduce all truth from it. As we have seen, Clark does not believe that we can deduce all truth from it. He believes that we only obtain knowledge from revelation. But suppose someone denies that the Trinity is logical. The Clarkian will point out that Scripture teaches that logic is the essence of correct thinking, and that the Trinity must therefore be explicable in purely logical terms. Logic is the essence of correct thinking, but it must subordinate itself to Scripture. The one who uses "logic" to attempt to prove that God is monadic rather than Triune is being illogical, from the Clarkian perspective, and he is suppressing the truth in unrighteousness. If such a person is using the Qur'an to justify this supposedly logical deduction, the Clarkian may demonstrate in a manner strikingly similar to that of a Van Tillian that the Muslim's epistemology is inconsistent and illogical. Unregenerate humans do sometimes use logical thought, but they do so inconsistently. The human is only consistently logical when he subordinates his logic to the propositions of the Bible, and to the extent that he does not do this, he is illogical.
Plato in his theory of reminiscence may have used a myth to describe the disembodied soul of man contemplating the Ideas directly. This might be taken to mean that each soul is omniscient. In the Christian system this is not needed. Perhaps I have said that truth is the whole; at least I have insisted that all truth form a consistent system and have their meaning as parts of that system. (Christian Philosophy, 2004, pgs. 144)
So Clarkians, like Van Tillians, accuse unbelievers of being logically inconsistent. Yet Clarkians argue that in order to bring the unbeliever into consistency, they must be converted and assent to the same logical system of propositions which God assents to. This does not mean that man will become omniscient. Only God is omniscient. What is odd about Van Tillians is that they will accuse unbelievers of being logically inconsistent only to offer a system of propositions which even by their own standards does not conform to a logically consistent set of propositions. For example, John Robbins quotes Van Til as saying “My concern is that the demand for non-contradiction when carried to its logical conclusion reduces God’s truth to man’s truth." Well then, if the Christian can be logically inconsistent, why is the unbeliever criticized for doing the exact same thing? If human logic is radically different from God's logic, the unbeliever may at times be thinking in a manner more similar to that of God than Van Tillian Christians! So much for thinking God's thoughts after Him!
We can summarize like this: For Van Tillians, what is at issue is primordial rationality (i.e., one's standards of intelligibility, consistency, correctness, etc. broadly construed). One's worldview dictates one's standards of rationality, truth, intelligibility, etc. Logic is not the essence of correct thinking. Rather, biblical truth is the essence of correct thinking. So any worldview which insists that it is, has reduced the wide variety of correct thinking furnished in the Bible to a particular species of thinking that is not universally valid. For Clarkians, the Bible itself says that logic is the essence of correct thinking. Therefore, Van Tillians think Clarkians are rationalists.