"Oh, the depth of the riches both of the wisdom and knowledge of God! How unsearchable are His judgments and unfathomable His ways!" This quotation is from Romans 11:33, and it describes the vastness of the mind of God. The terms "unsearchable" and "unfathomable" are used to show that God's judgments and ways are immeasurable by any ability of man.
Nevertheless, the mind of God is revealed to man by His Holy Spirit. Notice the description of the wisdom of God as spoken by the inspired apostles and prophets in 1Corinthians 2:10-13:
10For to us God revealed them through the Spirit; for the Spirit searches all things, even the depths of God. 11For who among men knows the thoughts of a man except the spirit of the man which is in him? Even so the thoughts of God no one knows except the Spirit of God. 12Now we have received, not the spirit of the world, but the Spirit who is from God, so that we may know the things freely given to us by God, 13which things we also speak, not in words taught by human wisdom, but in those taught by the Spirit, combining spiritual thoughts with spiritual words.
The "things freely given to us by God" do not entail the entirety of the mind of God, for it is impossible for finite man to grasp the infinite knowledge and wisdom of God. Instead, these are specific things that God has chosen to reveal to man which are understandable to him.
Just as God chose to reveal certain portions of His mind to men, He has also chosen to preserve these through the writings contained in the Bible. "This treasure in earthen vessels" (2Cor. 4:7) has been transferred from those who received it by divine inspiration (2Tim. 3:16; 2Pet. 1:20-21) to the rest of us by the written word. To this point, the apostle Paul wrote concerning his letters, saying, "By referring to this, when you read you can understand my insight into the mystery of Christ" (Eph. 3:4). Indeed, anyone can now know what Paul knew about "the mystery of Christ." Likewise, Romans 15:4 states, "For whatever was written in earlier times was written for our instruction, so that through perseverance and the encouragement of the Scriptures we might have hope." When Paul wrote this, he was certainly speaking of the Old Testament, but the principle of this verse now holds for the New Testament also, which we rightly classify as a part the Scriptures, including Paul's own writings (2Pet. 3:16). The whole of the Bible was written for our understanding, instruction, perseverance, encouragement, and hope.
Why didn't God reveal more than what is written in the Scriptures? This question is sometimes asked by both believers and skeptics alike. They wonder why God allows so much of life, both temporal and eternal, to remain a mystery to man. There are good answers for this question. First, as it has already been mentioned, the infinite mind of God is unsearchable for finite man. As David stated in Psalm 139:6, "Such knowledge is too wonderful for me; it is too high, I cannot attain to it." Indeed, many heavenly things are far above the mind of man in his current state. When Paul had his own vision of Paradise, he "heard inexpressible words, which a man is not permitted to speak" (2Cor. 12:3-4). Consequently, he did not attempt to write those words, for it was an impossible and impermissible task.
Another reason that God has not revealed more than what is written is because of His own sovereign choice. Man has no right to demand that God explain Himself or anything else. When God chooses to remain silent, we must respect His silence as much as we respect His word. In Deuteronomy 29:29, Moses gave an important principle concerning God's silence: "The secret things belong to the LORD our God, but the things revealed belong to us and to our sons forever, that we may observe all the words of this law." To this point, consider also the book of Revelation. At the beginning of his vision, the Lord told the apostle John, "Therefore write the things which you have seen, and the things which are, and the things which will take place after these things" (Rev. 1:19). At the conclusion of his vision, an angel said to him, "Do not seal up the words of the prophecy of this book, for the time is near" (Rev. 22:10). Yet John was not permitted to write of everything he saw. For example, in the midst of his vision he heard "seven peal of thunder" utter their voices (Rev. 10:3). When he was about to write what he had heard, a voice from heaven commanded him, "Seal up the things which the seven peals of thunder have spoken and do not write them" (Rev. 10:4). Why were these things kept secret? It was simply by the divinely wise choice of Almighty God, and so it is for many heavenly things.
Therefore, let us be delighted with the things that are written in the Scriptures, and let us be satisfied that there are some things that we may not know. God "has granted to us everything pertaining to life and godliness, through the true knowledge of Him who called us by His own glory and excellence" (2Pet. 1:3). Thus, we can know all that we need to know, and that should be enough to satisfy us. Indeed, the word of God is so rich with wisdom and knowledge that a lifetime of study is not sufficient to learn it all. It is simply foolish for anyone to suggest that the Bible is not enough for us. "So then do not be foolish, but understand what the will of the Lord is" (Eph. 5:17).
