Meditating on a verse from Jeremiah, I came to the realization that, in effect, the Bible teaches us that there's a pathological liar beating in every human breast:
The heart is deceitful above all things,There's nothing filled with more deception than the heart, and because the heart informs the totality of a man, man is constantly being deceived and misled by the desires of his heart.
and desperately sick;
who can understand it? [Jer. 17:9]
The heart is "desperately sick": The illness is chronic, the cancer of deceitful pervasive. "Who can understand it?", there's no human medicine or treatment to cure it.
Solomon concurs in Ecclesiastes, writing under the inspiration of the Holy Sprit:
3 This is an evil in all that is done under the sun, that the same event happens to all. Also, the hearts of the children of man are full of evil, and madness is in their hearts while they live, and after that they go to the dead. [Ecclesiastes 9:3]And who better than the Great Physician to list some symptoms of an evil heart:
20 And he said, “What comes out of a person is what defiles him. 21 For from within, out of the heart of man, come evil thoughts, sexual immorality, theft, murder, adultery, 22 coveting, wickedness, deceit, sensuality, envy, slander, pride, foolishness. 23 All these evil things come from within, and they defile a person.” [Mark 7:20-23]The prognosis isn't good at all: Your heart dreams up evil, you do that evil, then you die and face God's wrath against that evil. What's needed is a heart transplant, something only God can perform:
24 I will take you from the nations and gather you from all the countries and bring you into your own land. 25 I will sprinkle clean water on you, and you shall be clean from all your uncleannesses, and from all your idols I will cleanse you. 26 And I will give you a new heart, and a new spirit I will put within you. And I will remove the heart of stone from your flesh and give you a heart of flesh. 27 And I will put my Spirit within you, and cause you to walk in my statutes and be careful to obey my rules. [Ezekiel 36: 24-27]A changed heart sealed with the Holy Spirit is what fuels the Christian life of obedience to Christ. It's a heart changed and changing by the preaching of the Gospel and the inward working of the Holy Spirit that produces godly fruit in the Christian life.:
16 But I say, walk by the Spirit, and you will not gratify the desires of the flesh. 17 For the desires of the flesh are against the Spirit, and the desires of the Spirit are against the flesh, for these are opposed to each other, to keep you from doing the things you want to do. 18 But if you are led by the Spirit, you are not under the law. 19 Now the works of the flesh are evident: sexual immorality, impurity, sensuality, 20 idolatry, sorcery, enmity, strife, jealousy, fits of anger, rivalries, dissensions, divisions, 21 envy,  drunkenness, orgies, and things like these. I warn you, as I warned you before, that those who do such things will not inherit the kingdom of God. 22 But the fruit of the Spirit is love, joy, peace, patience, kindness, goodness, faithfulness, 23 gentleness, self-control; against such things there is no law. 24 And those who belong to Christ Jesus have crucified the flesh with its passions and desires. [Gal. 5:16-24]Any attempt on our part to trust in our own perceived goodness or righteousness or to boil down the Christian life to mere moralism is to listen to the insane, chronically ill pathological liar within.