Sufficiency
By sufficiency, Van Til didn't mean that there is no need for supernatural revelation. General revelation provided the background for supernatural revelation. For that purpose, general revelation is sufficient.
Three things consist in this idea of sufficiency. One, general revelation or nature is sufficient to show God's curse as a consequence of man's disobedience. Moreover, it is also sufficient to render man without excuse.
In this discussion of sufficiency, though the idea of nature crying for vengeance has been familiar to me, I think many will find this strange. This is the inevitable outcome when man abuse nature.
Perspicuity
In explaining the perspicuity of nature, Van Til shared us something profound by these statements:
The perspicuity of God's revelation in nature depends for its very meaning upon the fact that it is an aspect of the total and totally voluntary revelation of a God who is self-contained. God's incomprehensibility to man is due to the fact that he is exhaustively comprehensible to himself (p. 277).
If your head did not spin while reading the above two sentences, I congratulate you. As for me, the above sentences are tough and to be comprehensible requires a more detailed explanation, which to my mind is not part of Van Til's writing style. He assumes that his thoughts are clear and his readers understand them.
Anyhow, one thing is clear in Van Til's statement that perspicuity of nature "is an aspect of the total and totally voluntary revelation of God who is self-contained." In explaining this idea of a God who is self-contained, Van Til mentioned God's incomprehensibility to man. These ideas seem to contradict each other. What is the relationship between perspicuity, a God who is self-contained, and God's incomprehensibility?
Perhaps the best way to understand the relationship between the ideas above is to relate the idea of being "self-contained" to man. I understand a self-contained man as someone independent and self-sufficient. He does not need anyone, not even God. Understood from this sense, can we say that such a man exists? Perhaps yes, in his arrogance man can claim that he does not owe his existence from a higher being described in the Bible as "God". However, from a Christian perspective, such a man does not exist. There is no man that live on planet earth since its creation as someone self-contained. Such an attribute can only be rightfully claimed by God alone. If you are truly self-contained, then you might be "god" in the Christian sense of the word.
Even up to this point provided that we understand the meaning of self-contain correctly, still its connection to the perspicuity of nature isn't clear. I hope that the additional explanation of Van Til as to the incomprehensibility of God will help clarify this relationship.
God is incomprehensible to man and this is because "he is exhaustively comprehensible to himself." Again, can any man claim that he knows himself exhaustively? I doubt that. We don't know who we really are. Perhaps, we know ourselves better than others know us, but we cannot say that we know ourselves exhaustively. Only God possesses such exhaustive knowledge of Himself, of man, and of anything created in this world. And according to Van Til this is the reason why God is incomprehensible to us.
If the above explanation is still not clear, I think the doctrine of creation of man in God's image will assist us clarifying this matter. Two things are implied in this doctrine of creation:
One, since man is God's creation, he cannot possess an exhaustive knowledge of himself just like the kind of knowledge that God has. Man can pretend to have exhaustive knowledge of himself if he denies the doctrine of creation. The being and knowledge of the Creator is always far higher than man's being and knowledge.
Two, since man is created in God's image, though his knowledge is not exhaustive, it is true, clear, and certain. Man does not need exhaustive knowledge for his knowledge to be true, clear, and certain. I think the concept of analogical thinking is relevant to this point.
Mystery
Nevertheless, the claim to clarity of God's revelation does not mean the absence of mystery. Man as creature cannot penetrate to the very bottom of God's revelation in nature. There is always some aspect of nature that is beyond human comprehension simply because man's knowledge is limited.
Acceptance
Van Til concludes the discussion on the characteristics of natural revelation in the idea of acceptance from the perspective of the Westminster Standards. It is here that Van Til clarifies that no man under the power of sin can accept the revelation of God both in nature and in Scriptures. Man in sin cannot accept the God who is revealed in both revelation. He explains this rejection in relation to Christian theism:
From the point of view of the sinner, theism is as objectionable as is Christianity. Theism that is worthy of the name is Christian theism. Christ said no man can come to the Father but by him. No one can become a theist unless he becomes a Christian. Any God that is not the father of our Lord Jesus Christ is not God but an idol (p. 280).
The last part of this lecture will be about the natural theology of Greek origin.