The presuppositionalist cannot be a realist for the fact that he demands a mode of knowing that is simply not possible on realism. He demands the starting point of an idea or belief. But ideas are not things. He must also account for connecting thought to reality, which is not possible when thought and reality are walled off from each other.
If the presuppositionalist started from things and not ideas/beliefs, then his view would not get off the ground. Because to start from things would be to acknowledge that we come to know things through the senses, that we know truth by correspondence to reality, that we can know God through His effects, and that we know the Bible is true because we first know what truth is, what language means, and so forth. For example, how can we understand the Bible if we do not first know language or understand concepts derived from the apprehension of being as it is? So, we cannot truly start with presupposing the Bible because such a thing cannot in principle be done. The logical and sequential order of man’s natural mode of knowing is reversed on the presuppositionalist view. Per Van Til:
"We should accept the Scripture testimony about itself. If we did anything else we would not be accepting Scripture as absolute. The only alternative then to bringing in a God who testifies of himself and upon whose testimony we are wholly dependent, is not to bringing in God at all. And not to bring in God at all spells nothing but utter ruin for knowledge" A Survey of Christian Epistemology, p. 202
One accepting what Van Til says here, and in other places, cannot be a realist. The mode of knowledge posited by Van Til can only be understood within an idealist framework.
When the Christian claims that the Bible is true, we are claiming that the Bible corresponds to reality. We are claiming that what the Bible reports about events is true, what it tells us about God is true, and all the rest. Yet, how can we know something is true if the mode of human knowing is circumvented? This is precisely what the presuppositionalist is asking. He is essentially saying that the way we normally know things does not apply to the Bible or God. So, taking our que from the presuppositionalist, should the unbeliever suspect that his normal way of knowing things is delivering falsehood? It seems so. Yet, if this is the case, then why should he trust the normal way he is knowing the things that the presuppositionalist is telling him about God and the Bible?! Throwing shade at the unbelievers’ knowing faculties undermines the entire conversation. Further, arguing that a person’s knowing faculties are mixed with error is an argument for skepticism. And the only fertile ground for skepticism is in idealism. Moreover, one would be arguing from an epistemic elitism; the presuppositionalist holds that only they have proper knowing faculties to adjudicate truth and falsehood. This would then amount to an a priori denial of anything the opposing person says in disagreement.
What the presuppositionalist must also avoid is claiming that, once the Bible is presupposed as true, the truth of it will then be evident. This self-defeating rationale is employed in other religions (such as LDS). Further, I do not see any limit to the potential post-hoc rationalization or confirmation bias problems that can arise if it is asked that the Bible be presupposed as true in order to establish its truth.
Is the Bible true for the believer and unbeliever? Certainly, we want to say that it is. Thus, we must say that it is objectively true, and that the unbeliever may withhold assent or try to undermine its truth. In any event, there are propositions within the Bible which we assert correspond to mind-independent reality. And if we desire to maintain this correspondence, we cannot demand a bifurcated way of knowing things. So, do we know the Bible because the ideas of it are implanted in us? Or, do we know the Bible because we learn language, understand truth, then apprehend that the Bible is not contradicting itself, reports historical events, etc.? I think the latter is much more plausible.
What about God simply giving us knowledge of things? Surely, He could circumvent the natural knowing process and grant us knowledge of Himself directly (beaming it into our mind) or of the Scriptures. I would not necessarily disagree but would qualify my response, lest we saw off the branch upon which we are sitting. The only way we can be sure that God is revealing things to us is by having a reality by which we can understand such revelation. If God tells Noah to build an ark, Noah knows what an ark is, what rain is, what a flood is, and so forth. Noah understands and grasps language and meaning. And Noah understood this before God spoke to Him. God communicates to us in accordance with the type of beings we are. It would go against the nature of God to usurp how He created man and would go against His nature to think that He would create us to understand things in a certain way and then constantly override that when He needed to especially reveal something to us (whether an Old Testament patriarch or New Testament believer). I think God communicates to us in accordance with our nature. If God shows the Hebrew special instructions for metalworking within the Tabernacle, then this is understood as a human understand things. This is, of course, one of the reasons why the Bible is written in human language, for us to learn, pass along, teach from, and to treasure. Does the Holy Spirit tell us what the passages mean? Or does He guide us in our understanding, drawing is closer to the Lord and His Word? I think our sanctification is described in the Bible as a cooperative effort, and it is for our benefit that we become obedient. This does not at all diminish the spiritual life of the believer but makes it intelligible and veridical.
I suspect a major reason why the presuppositionalist must posit God on pain of never getting to God is because his idealism forbids him from knowing things in themselves. Baked into his philosophical anthropology is a tacit perceptual subjectivism. Without God as the transcendental, there be only nihilistic relativism because things cannot ultimately be known as they are. Thus, Christian proofs for God, as famously attacked by Kant, are vulnerable because the reasoning gap of effect to cause cannot be bridged. Positing God at the outset, and the Christian worldview as a transcendental framework, solves this problem. Again from Van Til:
That is, we must seek to determine what presuppositions are necessary to any object of knowledge in order that it may be intelligible to us. It is not as though we already know some facts and laws to begin with, irrespective of the existence of God, in order then to reason from such a beginning to further conclusions. It is certainly true that if God has any significance for any object of knowledge at all, the relation of God to that object of knowledge must be taken into consideration from the outset. It is this fact that the transcendental method seeks to recognize. - A Survey of Christian Epistemology p.201
It is clear how the idealist lays down his method before exploration as if he knows what he will encounter and how he will react and respond before he sets out. The backwardness of this approach is evident of a type of “taxi-cab” fallacy; one arbitrary dismisses how he arrived at his present location. As Gilson accurately states, the idealist puts forth a method as a precondition for philosophy instead of finding method in his philosophy (as the realist does). The idealist starting point “has neither the evidence of an axiom nor the value of a principle” (Gilson, Methodical Realism, p.85). Further, there are no compelling reasons to accept the major problems that Descartes, Kant, or others in the modern era, were trying to solve. The problems begetting idealism and the transcendental are demonstrable facades. And once we see these problems for what they are, the need for the transcendental method – and all of its attending problems - fades away.
I think the presuppositionalist could resort to saying, “God must regenerate you, then you will know the truth.” But how does the presuppositionalist know that he has been regenerated to know the truth? He would just be appealing to the Scriptures to establish the truth he has already presupposed as true. Or, how does he avoid the question of a demon or evil spirit giving him a false idea of regeneration? The presuppositionalist might also reply that only a regenerated person can understand the truth of the Scriptures and God, the unbeliever cannot. The Bible does say the things of God are foolishness to the impenitent man, but this does not mean the unbeliever cannot understand and apprehend what is being told to him; he simply rejects it as false based on myriad (futile, I think) grounds. And, of course, I agree that the unbeliever’s view is irrational. But, again, the way we establish the rationality of our view is of the utmost importance. The believer may understand the truths of the Scriptures in a much deeper, and entirely distinct way than the unbeliever. Think of an orphan reading a letter from what he took to be another person’s father, and then later discovering through various means that he was actually reading a letter from his own father. The letter would mean much more on the second reading.
Moving away from idealism is important for Christians. Upholding the divide between object and subject, whether explicitly or implicitly, brings intractable problems for the Christian worldview. We must maintain that what man knows are things. Thus, the point of departure and proper starting point for philosophy is being. There is nothing to know if there are not things. Getting this out of order led to the problems that the presuppositionalist sees as highly relevant. These problems need not have arisen in the first place, so continuing to give them life and constructing philosophical and apologetic systems around them represents time that could be better spent elsewhere.
I realize the presuppositional question and method is closely tied to Reformed Theology. And those strongly committed to that tradition will bristle at the points raised above. Still, we should seek a unified worldview and biblical prolegomena. Inconsistency must be rooted out where we find it.