The heart of a person can be corrupt, or it can be good and helpful, depending on what has been put into it, and whether it is being led by the truth. Human beings are created in the image of God with the faculty of reason. In the fallen world, with fallen flesh, ruled by the prince of darkness, to be led by the heart by default is beyond foolish.
The heart is a conglomeration of what has been put into it, and part of the discipleship process to “be transformed by the renewing of your mind. Then you will be able to test and approve what God’s will is—his good, pleasing and perfect will” (Ro 12:2). By renewing the mind, the heart is purged of evil and falsehood and filled with truth. One way to be led by the deceitful heart is to give no energy to seeking the Kingdom of God and his righteousness. To seek the Kingdom is to consider the godliness of one’s thoughts and actions, abstaining from those which are not of God, and engaging in those that are. In this way, the heart will become a useful tool. The emotions will serve the believer rather than leading the believer into foolishness.
God promises through the prophet Ezekiel to give a new heart—a heart of flesh instead of a heart of stone (Ez 36:26), and he says that he will write his laws on our hearts (Jer 31:33) so that we will know him and follow his ways. Were it not for God’s intervention, then the heart would indeed be beyond all cure.
Breaking Down the Key Parts of Jeremiah 17:9
#1 “The heart…”
This can refer to many things, but in this context, it is the seat of the soul, the emotions, and the desires.
#2 “…is deceitful above all things…”
Being led by the heart is to be led by lusts, or emotionalism. In the relationship between the heart and the mind, particularly the renewed mind, it is the mind that should lead the heart. As a follower, the heart is useful and adds to the richness of life. Emotions are a good thing if put in their proper place. They can be an accurate reflection of what is happening. The well-led heart can enrich love, joy, godly grief, and every other emotion in its proper context. But if it is allowed to lead an unregenerate mind, or even a renewed, but lazy mind, it will lead astray into foolishness or sin.
#3 “…and beyond cure.”
This is true of the heart apart from God’s intervention. But God will give a new heart and write his laws on it through the process of salvation and sanctification.
#4 “Who can understand it?”
Understanding the physical anatomy and function of the organ that is the human heart is infinitely easier than understanding the spiritual/emotional aspects. Theologians and human behaviorists struggle to understand it, but God, who designed it all, understands it perfectly and has made a way to cleanse even the most deceitful of hearts through the blood of his Son, Jesus Christ.