It may be that you are a Christian who is weak, uncommitted. You know Christ. You have come to Calvary. You have repented of your sin, you have known an unmistakable experience of conversion, but maybe you have not advanced much further. So you are weak as a Christian, and Christ is only an occasional help to you. Yes, you may pray every day, briefly, you may read the word, but your life is not really his, so you go to work or study and you do many worldly things. When you want to buy a new car you buy the best you can, you do everything to suit you. You never think, should I not seek the guidance of the Lord and the principles of the Scripture and put my life for him first. You have no avenue of service for the Lord. All your time, rather like a worldling, is fully engaged in your career prospects, your advances, your home, and in feathering your nest. There is a nice side to you, you do love the Lord. You do come to church, maybe once, maybe even twice in a week, but you are not really living a committed Christian life. Then you cannot know the deep things of God. That is what the Scripture says, because your spiritual faculties, your senses, are not ‘gymnasticised’; they are not exercised in proving the Lord, in seeing his point of view, in seeking his guidance and being at his disposal. You do not put him first before career or home, so you cannot understand says this passage.
Equally there are some people and they are very enthusiastic about their Christian life at an intellectual level and study the books and go to meetings and they are ever learning. They learn the doctrines of grace, and the theology of the Christian life, and yet they are quite worldly people and they please themselves. They skip around and there is no real commitment or dedication. The Christian faith is all an intellectual game almost. They may be saved. There may be fundamental elements of Christian living going on in their lives: some prayers, some reading of the Scripture, but once again the life is not really committed to God. Consequently, they cannot really grasp those things, except at a kind of theoretical level. They cannot really appreciate those things, their view of Christ will never grow, their experience of God will never deepen because their priorities are all wrong and their spiritual faculties are not exercised in knowing good from evil. They go home and play worldly pop to themselves and walk around with it on their smart phones and so on they are half worldlings. They are not really testing what is right and what is wrong and so their spiritual faculties are not exercised. It is a remarkable chapter: it brings us closer to the Lord and encourages us to be ever making progress in the Christian life.