The Deceitfullness of the Human Heart

Jeremiah 17:9 The heart is deceitful above all things and beyond cure. Who can understand it?

Jeremiah 17: 5-11
5 This is what the Lord says:

“Cursed is the one who trusts in man,

who depends on flesh for his strength

and whose heart turns away from the Lord.

6He will be like a bush in the wastelands;

he will not see prosperity when it comes.

He will dwell in the parched places of the desert,
in a salt land where no one lives.

7“But blessed is the man who trusts in the Lord,
whose confidence is in him.

8He will be like a tree planted by the water
that sends out its roots by the stream.

It does not fear when heat comes;

its leaves are always green.

It has no worries in a year of drought

and never fails to bear fruit.”

9The heart is deceitful above all things

and beyond cure. Who can understand it?

10“I the Lord search the heart

and examine the mind,

to reward a man according to his conduct,

according to what his deeds deserve.”

11Like a partridge that hatches eggs it did not lay

is the man who gains riches by unjust means.

When his life is half gone, they will desert him,

and in the end he will prove to be a fool.

Here's a commentary on Jeremiah 17:9, which I enjoyed:
Gills expository:
Jeremiah 17:9
who can know it? angels do not, Satan cannot; only the spirit of a man can know the things of a man within him; though the natural man does not know the plague of his own heart; the Pharisee and perfectionist do not, or they would not say they were without sin; such rant arises from the ignorance of their own hearts; only a spiritual man knows his own heart, the plague of it, the deceitfulness and wickedness in it; and he does not know it all; God only knows it fully.

Mathew Henry's Concise Commentary:
Jeremiah 17:5-11:

He who puts confidence in man, shall be like the heath in a desert, a naked tree, a sorry shrub, the product of barren ground, useless and worthless. Those who trust to their own righteousness and strength, and think they can do without Christ, make flesh their arm, and their souls cannot prosper in graces or comforts. Those who make God their Hope, shall flourish like a tree always green, whose leaf does not wither. They shall be fixed in peace and satisfaction of mind; they shall not be anxious in a year of drought. Those who make God their Hope, have enough in him to make up the want of all creature-comforts. They shall not cease from yielding fruit in holiness and good works.

The heart, the conscience of man, in his corrupt and fallen state, is deceitful above all things. It calls evil good, and good evil; and cries peace to those to whom it does not belong. Herein the heart is desperately wicked; it is deadly, it is desperate. The case is bad indeed, if the conscience, which should set right the errors of other faculties, is a leader in the delusion. We cannot know our own hearts, nor what they will do in an hour of temptation. Who can understand his errors? Much less can we know the hearts of others, or depend upon them.

He that believes God's testimony in this matter, and learns to watch his own heart, will find this is a correct, though a sad picture, and learns many lessons to direct his conduct. But much in our own hearts and in the hearts of others, will remain unknown. Yet whatever wickedness there is in the heart, God sees it. Men may be imposed upon, but God cannot be deceived. He that gets riches, and not by right, though he may make them his hope, never shall have joy of them. This shows what vexation it is to a worldly man at death, that he must leave his riches behind; but though the wealth will not follow to another world, guilt will, and everlasting torment. The rich man takes pains to get an estate, and sits brooding upon it, but never has any satisfaction in it; by sinful courses it comes to nothing. Let us be wise in time; what we get, let us get it honestly; and what we have, use it charitably, that we may be wise for eternity.