To have knowledge, in a robust biblical sense, is not merely to assent to certain propositions because they are true, but also to understand (1) their relations as parts to a whole, and (2) the ends toward which they are oriented:
I know that thou canst do all things, and that no purpose of thine can be restrained. “Who is this that hideth counsel without knowledge?” Therefore have I uttered that which I understood not; things too wonderful for me, which I knew not. (Job 42:2–3)
No doubt Job knew what had been happening to him, in the philosophical sense — he had beliefs which were justified because they stood in a proper causal relation to the events. But here he acknowledges two additional things:
The “doings” which he has been trying to understand are God’s; i.e., all the facts are integrated into the will of God: they are parts related to the whole under the arche of Christ.
All these facts have an end or telos (“purpose”).
It is because Job has tried to give an account for what has happened to him without “understanding” how these facts stood in relation to the fact of his own holiness; nor the end toward which they were oriented; that his counsel has been “without knowledge.” Indeed, this knowledge is too “wonderful” for him; in Hebrew, too “accomplished;” i.e., beyond his power to grasp.
In this sense, Job had a philosophical knowledge of what had happened, but not a biblical knowledge. Or, I think it would be better to say that he had a natural knowledge, but not a spiritual knowledge.
Thus Proverbs 1:7 says, “The fear of Yahweh is the beginning (Heb. “head”) of knowledge; wisdom and instruction hath fools despised.” To truly know in a biblical sense requires starting with submission to the integration point — the head, the arche as the New Testament would say: God in Christ. True knowledge is not merely an accurate mental representation of raw facts; it is a right understanding of their place in the essentially teleological hierarchy of being.
This is why knowledge in scripture is often described in participative terms. The most potent example is the man “knowing” his wife (Gen 4:1 etc). But similarly, in Exodus 35:31, “knowledge” is paralleled with “workmanship;” it is not merely an abstract mental model of facts, but a practical ability to participate in them: to enter into the hierarchy of being with respect to “work in gold, and in silver, and in brass, and in cutting of stones for setting, and in carving of wood” (Ex 35:32–33). It is a unification between the spiritual and the physical. By contrast, the man who accidentally kills someone is described as doing so literally “lacking knowledge” (Dt 4:42; 19:4): his spirit was not meaningfully participating in directing the end of the physical event.
This participation of the physical in the spiritual is key to understanding how the heavens can “show knowledge” about God’s eternal attributes (Ps 19:2); a knowledge which is “visible” to everyone (Rom 1:19). The nature of the physical heavens reveals the nature of the spiritual heavens directly to the soul. But souls in rebellion against their creator refuse to participate in this knowledge, because this would mean submitting to the hierarchy of being: bringing the knowledge into themselves, and being raised up by it, so they may be integrated into God. Rather than be held down by knowledge in this way, they rather “hold down” the knowledge itself in unrighteousness (Rom 1:18) — so they can continue their intentional disintegration from God. It is because they will not fear the Lord that they cannot submit even to this rudimentary knowledge.
Note, however, that they can indeed receive it; in fact, it is impossible for them not to. God has so ordered things that these facts are “plainly seen,” so they are “without excuse” (Rom 1:20). The problem is not mental, but moral: they are unwilling to participate with the truth; not unable to see it. To put it slightly differently, they can form the justified belief that the Everlasting Almighty exists; but they will not conform themselves to it or act in accordance with it.
This is where it is both right and wrong to speak of justified belief being impossible apart from Christ:
All truth is from Christ, through Christ, and to Christ: he is its origin, its revealer, and its end (Col 2:3; Rom 11:36). Thus, any truth can only be apprehended — whether by a believer or an unbeliever — because of the prior existence, work, and nature of God the Son. An unbeliever who accurately measures and believes the airspeed velocity of an unladen European swallow is in reality receiving from Christ a truth that is integrated into, and oriented toward himself. So he is engaged in a performative contradiction: he knows the fact, because it is true; but he denies the Son, who makes it true. His belief is justified empirically; but unjustified spiritually. He receives it from Christ; but holds it apart from Christ. He knows what the fact is; but not why it is. He wrenches the swallow and its nature out of the hierarchy of being in which it truly participates, with Christ as its integrating principle — and tries to present measurements about it as brute facts with no integrating principle; or give some other explanation of their origin and meaning.
In the end, then, everything he says is untrustworthy, because he is actively trying to disintegrate truth from the arche of Christ in which it holds together. It is not that he is unable to discover and know true things. It is that he continually refuses to rightly order what he comes to know toward its origin and end, because “every form of the designs of his heart are only bad all the day” (Gen 6:5).
Should we trust such a mind?
Well, he can have some knowledge that we can learn from — in the basic and emaciated sense of externally justified beliefs. He may even gain some knowledge in the biblical sense of participative understanding — but it will always stop short of knowledge in the fullest or truest sense, because he will always stop short in the hierarchy of being, refusing to go all the way to the top, so that he may integrate everything he knows into Christ. He will always find some other integration point lower down — the very definition of idolatry.
This makes him innately untrustworthy. He can make accurate measurements and valid inferences — if he wants to. But whether he wants to depends entirely on whether he can do so while denying Christ. So we always have to be looking for how his religious commitments could be affecting his process and tainting his knowledge. We certainly shouldn’t be deferring uncritically to him as an “expert” — for the only true expert is one who can take everything he knows, and integrate at least the major parts of it into a coherent whole under the arche of Christ.
I’m not sure the van Tillian ‘everything the unbeliever says is ultimately untrustworthy’ is fruitful or useful long term. It’s rhetorically powerful, and useful within the specific theological and metaphysical framework you’ve provided, but functionally, long term, ends in creating a Christian ghetto of knowledge where we don’t fully acknowledge the true expertise of others, something Calvin warned against, that Christians would be ignorant of the true knowledge of unbelievers.