The apostle Paul quoted from Isaiah 64:4 to show that we can never know the mind of God by our own abilities. "Eye hath not seen, nor the ear heard, the things which God hath prepared for them that love him" (1 Corinthians 2:9). He reminds us that a man cannot know the things within another's mind, without being told. Only his own spirit can know his thoughts. Likewise, a man cannot know what is in the mind of God. It is God's own Spirit that knows His mind. The point being emphasized by the apostle is that all we can possibly know about the will of the Lord is what has been revealed by the Holy Spirit to the apostles and prophets (Ephesians 3:5). We learn the things of God through the inspired revelation of Scripture (2 Timothy 3:16-17).
Religious discussions may sometimes prompt speculative questions: "What if...Don't you think...Is it possible...?"etc. Questions which seek a biblical answer are legitimate, but those which call for mere opinion may be useless. In fact, opinions which suggest and encourage alternatives to what God has told us can cause great harm. 1 Timothy 6:4 cautions against "doting about questions and strifes of words." Again, Paul says, "But foolish and unlearned questions avoid" (2 Timothy 2:23).
Those who want to appear scholarly may be tempted to answer unanswerable questions. Some may even speculate on ways to avoid what "Thus saith the Lord." A faithful teacher of the word, however, will be satisfied to point to the Scriptures and say, "That is all I know about it."
For example, numerous questions arise over divorce and remarriage. Admittedly there are sometimes difficult and complicated situations. But when all the "What ifs" and "It seems like" have been stated, the fact remains that what the Bible says is all I know about it. The problem what Jesus said in Matthew 19:9 is not that it cannot be understood. The problem is that people do not want to accept it.
So also in discussions about the plan of salvation. Questions arise about the honest seeker who dies without obeying the gospel. Arguments may be made with both emotion and reason. But when I have read the commands, promises and warnings in the Bible, that's all I know about it.
We must be careful not to think of men "above that which is written" (1 Corinthians 4:6). There is much emphasis on education, intelligence and ability. These may help in the study and teaching of the word, but it must be remembered that all spiritual knowledge must still start and end at the same place -- in the Bible.
One may know (or thinks he knows) a great many things about the material world. Such may be learned from many sources. But the only spiritual truth that I will ever know this side of eternity will be what I learn from the Bible. The bottom line is, "Here is what the Bible says. That is all I know about it!"