Thus saith the LORD; Cursed be the man that trusteth in man, and maketh flesh his arm, and whose heart departeth from the LORD. For he shall be like the heath in the desert, and shall not see when good cometh; but shall inhabit the parched places in the wilderness, in a salt land and not inhabited.
Blessed is the man that trusteth in the LORD, and whose hope the LORD is. For he shall be as a tree planted by the waters, and that spreadeth out her roots by the river, and shall not see when heat cometh, but her leaf shall be green; and shall not be careful in the year of drought, neither shall cease from yielding fruit.
The heart is deceitful above all things, and desperately wicked: who can know it? I the LORD search the heart, I try the reins, even to give every man according to his ways, and according to the fruit of his doings. As the partridge sitteth on eggs, and hatcheth them not; so he that getteth riches, and not by right, shall leave them in the midst of his days, and at his end shall be a fool.
– Jeremiah 17:5-11
The point about deceitfulness of hearts is not that every individual human heart is deceitful and incurable. There are hearts that trust in the Lord; they are blessed and flourish. But there are also hearts that trust in men and depart from the Lord. Saying “the heart is deceitful” is like saying “men are liars.” Both are generally true. Neither is true of every individual. And because men are liars, sorting out the honest from the dishonest is tricky work. Who can do it? But there is a judge in heaven who searches the hearts and will render to every man according to his works; to those who by patient continuance in well-doing seek for glory and honor from God, immortal life; to those who disobey the truth and trust in men, wrath.