The corruption of the heart begins with its inherent deceitfulness above all things and desperate wickedness, rendering it subtle, false, and apt to supplant truth by calling evil good and good evil while crying peace where none exists. This inward condition, often hidden even from its owner, generates evil devices of pride, ambition, and every form of corruption, leading individuals to self-deception and ruin through suggestions that there is no God who sees or requires obedience. The old heart, marked by human nature, resists burial and functions like a coiled spring that surges upward the moment attention shifts from God, producing stubbornness, jealousy, hatred, lust, superiority, selfishness, fear, and excuses that justify neglecting prayer, study, and righteous conduct. Such an attitude originates in the mind or heart and shapes every approach to life, determining whether conduct reflects the corrupt state or the new heart God supplies. Guarding the heart with all diligence is therefore essential, since the issues of life flow from it, and the flesh profits nothing apart from the Spirit that imparts life through Christ's words. Failure to do so results in gradual hardening, unbelief, and departure from the living God, as seen when people drift from initial zeal into neglect of their calling and eventual rebellion. This internal conflict requires continual discipline to subdue the old self, lest one become disqualified after instructing others, and demands that every effort remain directed toward the imperishable crown rather than the perishable rewards of the world.

The heart is deceitful above all things and desperately wicked, as stated in Jeremiah 17:9. The Hebrew term rendered deceitful derives from the same root as the name Jacob and appears only three times in the Old Testament. In reference to human character it denotes prideful vanity that puffs up the person, together with fraudulence that intentionally perverts truth in order to induce surrender of what is valuable. The same term is translated corrupted and polluted in its other occurrences, indicating something foul that belongs in the sewer. The word rendered desperately describes a condition so weak and frail that it is at the point of death; modern renderings therefore often substitute incurable. This heart of stone resists penetration by truth and remains fixed in a pattern that cannot be improved. Because the heart is incurably self-centered, self-absorbed, and narcissistic, it cannot consistently obey the two great commandments to love God with all one's faculties and to love others as oneself. Any elevation of self to equal standing with God constitutes idolatry. Consequently no character of value to God's kingdom can be formed while this heart remains. Its nineteen characteristic expressions listed in II Timothy 3:1-5—beginning with love of self and including covetousness, boasting, pride, blasphemy, disobedience, ingratitude, absence of natural affection, implacability, slander, incontinence, fierceness, treachery, recklessness, high-mindedness, and love of pleasure rather than love of God—constitute varying intensities of the same underlying corruption. These qualities already existed in Paul's day and will intensify as the culture approaches conditions like those before the Flood. Within the church the same heart can regain influence, producing the Laodicean state in which converted people declare themselves rich and in need of nothing while rejecting the continuing provision of Christ. The result is a return to self-absorption that severs the relationship requiring sacrifice. God therefore supplies His Spirit, which is the spirit of power, love, and a sound or disciplined mind. This Spirit enables control of the deceitful heart, equips believers for maturity, and prepares them to solve the problems of the coming Kingdom rather than allowing the heart's propensities to dominate.

The heart contains knowledge so deep that it cannot be comprehended, rendering it vulnerable and prone to deception rather than inherently deceitful or wicked. This condition leaves the inner person unaware of its own secrets and those of others, allowing external influences to shape thoughts, emotions, will, and actions without detection. Because the heart produces the issues of life, its vulnerability means that evil thoughts, adulteries, fornications, murders, thefts, covetousness, wickedness, lewdness, an evil eye, blasphemy, pride, and foolishness proceed from within and defile a person. God searches the heart and tests the mind to judge every individual according to ways and deeds, revealing that outward appearance cannot conceal the true inner state. Those who depart from the Lord suffer a curse while those who trust in Him receive blessing, yet even the faithful remain susceptible to deception unless the heart is deliberately prepared. Preparation requires setting the heart firm through honest confession, continual repentance, fervent prayer, careful meditation on God's Word, and fellowship with those of like mind so that it becomes steadfast and able to respond rightly. Guarding the heart with all diligence therefore demands rejecting perverse speech, fixing the eyes straight ahead, pondering the path of the feet, and refusing to turn aside, allowing God's peace to protect the inner person through trust in Him rather than one's own understanding.