Epistemology is almost always a complicated subject. To complicate matters, [Cornelius] Van Til is not easy to understand, and both [Gordon] Clark and Van Til do not speak at the same level and use the same terms differently. So in an effort to simplify things, I offer this as my understanding of the issue. [Those who want some documentary proof check out my other posts on Clark or Van Til]
According to Van Til, the Trinity is the archetype for all knowledge. According to Clark, the Trinity is a vital part of revelation but not the archetype for knowledge. Rather, the Scriptures is the principium and basis for all knowledge.
In Vantillian epistemology, since the Trinity is the archetype for knowledge, therefore all knowledge is by definition analogical. This is because there is a qualitative difference between God and Man with no univocal point of contact, as the Creator is distinct from the creature. All of Man's knowledge is creaturely and therefore cannot be univocal. "Univocal knowledge" in the Vantillian system refer to knowledge that erases the Creator/creature distinction and make the knower knowing truth as God knows it. Such a univocal knower may not know everything that God knows, but the part of the truth that he knows, he knows it to the same comprehensive detail that God knows that truth.
In Clarkian epistemology, which I am convinced IS the biblical and Reformed epistemology, we definitely agree that the Trinity is basic for Christianity. But the Trinity is basic in terms of ontology, in terms of grounding the world and everything that exists in its being. God in the Trinity is the principium essendi (beginning of being), and therefore without God nothing exists least of all [the being of] knowledge.
On top of this however, we realize that the Trinity is revealed to us humans by the Scriptures. Apart from Special Revelation in the Scriptures, no one can know the Trinity. The Trinity is not something that even a Christian can know intuitively and mystically apart from reading the Scriptures and internalizing the biblical data and worldview. Since this is the case, the Trinity cannot be properly basic for epistemology, for the Scriptures is the source for our knowledge of the Trinity.
The Scriptures is God's ectypal revelation to us. Since it is already accommodated to us humans, knowledge is univocal between God's intended revelation (ectypal theology simpliciter dicta) and our grasped knowledge (in subiecto). We can only know what God reveals. Even the Trinity is known to us only inasmuch as Scripture reveals it to us, either overtly or by good and necessary consequence.
As it can be seen, the two systems start off from very different foundations, and the divergence shows up in confusion as the two sides often misunderstand the other. I offer this of course to aid in some small manner to clear the fog.
Why do I not hold to Van Til's epistemology? As mentioned, I do not believe that the Trinity can be known apart from Scripture. I believe in a two track system differentiating between ontology and epistemology, with the Triune God as the principium essendi (beginning of being) and Scriptures as the principium congnoscendi (beginning of knowledge), and the two do not collapse into each other. Van Til's epistemology is also very problematic in making room for speculation on the Trinity, although of course Vantillians do not necessary do so. But if we place the Trinity prior to Scriptures, then why should our theories about the Trinity be necessarily tied to Scripture, since the former after all determines the latter in the Vantillian system? What safeguard is therefore in the Vantillian epistemology to stop speculations concerning the internal essence of the Trinity for the promotion of theories such as Social Trinitarianism, or making the Trinity the archetype for whatever three-fold relations we have concocted?
Such is why I am a Clarkian not a Vantillian.