What is taught here depends on what is gone before; and therefore they ought to be read together. Many lay hold on these words and mutilate them without understanding the design of the Prophet. This is very absurd: for we ought first to see what the prophets had in view, and by what necessity or cause they were led to speak, what was their condition, and then the general doctrine that may be gafilered from their words. If we wist to read the prophets with benefit, we must first consider the reason why a thing is spoken, and then elicit a general doctrine. Thus we shall be able rightly to apply this passage to a common use, if we first understand why the Prophet said, that the heart of man was insidious. He wished, no doubt, to be more earnest with the Jews; for he saw that they had so much wantonness and obstinacy, that a simple and plain doctrine would not have penetrated into their hearts. The declaration, that they are accursed who trust in men, and that no blessedness can be expected except we rely on God, ought to have been sufficient to move them; but when he saw that there was no sufficient power in such a declaration, he added, "I see how it is, the heart is wicked and vicious; so ye think that you have so much craftiness, that ye can with impunity deride God and his ministers: I, says Jehovah, I will inquire and search; for it belongs to me to examine the hearts of men." We hence see that there is an implied reproof, when he says, that the heart is insidious and wicked; [1] as though he had said, "Ye think yourselves in this instance wise; is not God also wise?" Isaiah says ironically the same, "Woe to them who go down to Egypt and make secret covenants, and who trust in horses, as though they could deceive me: ye are wise, I also have a portion of wisdom." (Isaiah 31:1) Notice especially the expression, "Ye are wise, etc.;" that is, "Ye are not alone wise; leave to me some portions of wisdom, so that I may be wise like yourselves." So also in this place, "Ye are deceitful and insidious, and think that I can be deceived:" for astute men are ever pleased with their own counsels, and seek to deceive God with mere trumperies. "Ye are," he says, "very cunning; but I, Jehovah, will search both your hearts and your reins." I cannot finish the whole to-day.

Footnotes

1 - The early versions and the Targum are neither consistent nor satisfactory as to the beginning of this verse: "Deep is the heart above all things, and it is man," Septuagint; "Depraved is the heart of all, and inscrutable," Fulgate; "Hard in heart is man above all things," Syriac; "The heart, deeper than anything, is human," Arabic; "Deceitful is the heart above all things, and it is strong." Targum. Correct, no doubt, is the first clause in the Targum, but not the last. Critics agree as to the first word, "deceitful," but not as to the word rendered in our version "desperately wicked." It occurs in all nine times, and four times in other parts of Jeremiah, (Jeremiah 15:18; Jeremiah 17:16; Jeremiah 30:12, 15) and it is rendered "incurable," except in Jeremiah 17:16. It means to be so bad as to be past endurance or past remedy. Blayney renders it here, "past all hope;" and Horsely, "incurable," which is perhaps the best word, -- Deceitful the heart above every thing, And incurable it is, who can know it? The meaning is, that it is incurably deceitful; hence the question," Who can know it?" -- Ed.

The train of thought is apparently this: If the man is so blessed Jeremiah 17:7-8 who trusts in Yahweh, what is the reason why men so generally "make flesh their arm"? And the answer is: Because man's heart is incapable of seeing things in a straightforward manner, but is full of shrewd guile, and ever seeking to overreach others.
Desperately wicked - Rather, mortally sick.

The heart is deceitful - עקב הלב akob halleb, "the heart is supplanting - tortuous - full of windings - insidious;" lying ever at the catch; striving to avail itself of every favorable circumstance to gratify its propensities to pride, ambition, evil desire, and corruption of all kinds.
And desperately wicked - ואנש הוא veanush hu, and is wretched, or feeble; distressed beyond all things, in consequence of the wickedness that is in it. I am quite of Mr. Parkhurst's opinion, that this word is here badly translated as אנש anash is never used in Scripture to denote wickedness of any kind. My old MS. Bible translates thus: - Schrewid is the herte of a man: and unserchable: who schal knowen it?
Who can know it? - It even hides itself from itself; so that its owner does not know it. A corrupt heart is the worst enemy the fallen creature can have; it is full of evil devices, - of deceit, of folly, and abomination, and its owner knows not what is in him till it boils over, and is often past remedy before the evil is perceived. Therefore trust not in man whose purposes are continually changing, and who is actuated only by motives of self-interest.

(i) The heart [is] deceitful above all [things], and desperately wicked: who can know it?
(i) Because the wicked always have some excuse to defend their doings he shows that their own lewd imaginations deceive them and bring them to these hardships: but God will examine their deeds by the malice of their hearts, (1-Samuel 16:7, 1-Chronicles 28:9, Psalm 7:10, Jeremiah 11:20, Jeremiah 10:12, Revelation 2:13).

The heart is deceitful above all things,.... This is the source of the idolatry and creature confidence of the Jews, sins which were the cause of their ruin; and though what is here said is particularly applicable to their hearts, yet is in general true of the heart of every man; which is "deceitful", and deceiving; and puts a cheat upon the man himself whose it is: it deceives him with respect to sin; it proposes it to him under the notion of pleasure; it promises him a great deal in it, but does not yield a real pleasure to him; it is all fancy and imagination; a mere illusion and a dream; and what it gives is very short lived; it is but for a season, and ends in bitterness and death: or it proposes it under the notion of profit; it promises him riches, by such and such sinful ways it suggests; but, when he has got them, he is the loser by them; these deceitful riches choke the word, cause him to err from the faith, pierce him through with many sorrows, and endanger the loss of his soul: it promises honour and preferment in the world, but promotes him to shame; it promises him liberty, but brings him into bondage; it promises him impunity, peace, and security, when sudden destruction comes: it deceives him in point of knowledge; it persuades him that he is a very knowing person, when he is blind and ignorant, and knows nothing as he ought to know; and only deceives himself; for there is no true knowledge but of God in Christ, and of a crucified Christ, and salvation by him; see 1-Corinthians 3:18 it deceives in the business of religion; it makes a man believe that he is a very holy and righteous man, and in a fair way for heaven, when he is far from that, and the character it gives him; in order to this, it suggests to him that concupiscence or lust, or the inward workings of the mind, are not sin; and it is only on this principle that it can be accounted for, that Saul, before conversion, or any other man, should be led into such a mistake, as to conclude that, touching the righteousness of the law, he was blameless: it represents other sins as mere peccadillos, as little sins, and not to be regarded; and even puts the name of virtue on vices; profuseness and prodigality it calls liberality, and doing public good; and covetousness has the name of frugality and good economy: it directs men to compare themselves and their outward conduct with others, that are very profane and dissolute; and from thence to form a good character of themselves, as better than others; and as it buoys up with the purity of human nature, so with the power of man's freewill to do that which is good, and particularly to repent at pleasure; and it puts the profane sinner upon trusting to the absolute mercy of God, and hides from him his justice and holiness; and it puts others upon depending upon the outward acts of religion, or upon speculative notions, to the neglect of real godliness; see James 1:22. The man of a deceitful heart, the hypocrite, tries to deceive God himself, but he cannot; he oftentimes deceives men, and always himself; so do the profane sinner, the self-righteous man, and the false teacher; who attempts to deceive the very elect, but cannot; yea, a good man may be deceived by his own heart, of which Peter is a sad instance, Matthew 26:33. The heart is deceitful to a very great degree, it is superlatively so; "above all", above all creatures; the serpent and the fox are noted for their subtlety, and wicked men are compared to them for it; but these comparisons fall short of expressing the wicked subtlety and deceit in men's hearts; yea, it is more deceitful to a man than the devil, the great deceiver himself; because it is nearer to a man, and can come at him, and work upon him, when Satan cannot: or "about", or "concerning all things" (q); it is so in everything in which it is concerned, natural, civil, or religious, and especially the latter. The Septuagint version renders it "deep"; it is an abyss, a bottomless one; there is no fathoming of it; the depths of sin are in it; see Psalm 64:6 and, seeing it is so deceitful, it should not be trusted in; a man should neither trust in his own heart, nor in another's, Proverbs 28:26, "and desperately wicked": everything in it is wicked; the thoughts of it are evil; the imaginations of the thoughts are so; even every imagination, and that only, and always, Genesis 6:5 the affections are inordinate; the mind and conscience are defiled; the understanding darkened, so dark as to call evil good, and good evil; and the will obstinate and perverse: all manner of sin and wickedness is in it; it is the cage of every unclean bird, and the hold of every foul spirit; all sin is forged and framed in it; and all manner of evil comes out of it, Revelation 18:1 yea, it is wickedness itself, Psalm 5:9, it is so even to desperation; it is "incurably wicked" (r), as it may be rendered; it is so without the grace of God, and blood of Christ:
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, as is expressed in the next words, which are an answer to the question; see 1-Corinthians 2:11.
(q) "de omnibus", vid. Noldium, p. 548. (r) "et immedicabili malo affectum", Gussetius; "incurabiliter aegrum", Cocceius.

deceitful--from a root, "supplanting," "tripping up insidiously by the heel," from which Jacob (Hosea 12:3) took his name. In speaking of the Jews' deceit of heart, he appropriately uses a term alluding to their forefather, whose deceit, but not whose faith, they followed. His "supplanting" was in order to obtain Jehovah's blessing. They plant Jehovah for "trust in man" (Jeremiah 17:5), and then think to deceive God, as if it could escape His notice, that it is in man, not in Him, they trust.
desperately wicked--"incurable" [HORSLEY], (Micah 1:9). Trust in one's own heart is as foolish as in our fellow man (Proverbs 28:26).

To bring this truth home to the people, the prophet in Jeremiah 17:9 discloses the nature of the human heart, and then shows in Jeremiah 17:10 how God, as the Searcher of hearts, requites man according to his conduct. Trust in man has its seat in the heart, which seeks thereby to secure to itself success and prosperity. But the heart of man is more deceitful, cunning than all else עקב, from the denom. עקב .moned , to deal treacherously). אנוּשׁ, lit., dangerously sick, incurable, cf. Jeremiah 15:18; here, sore wounded by sin, corrupt or depraved. Who can know it? i.e., fathom its nature and corruptness. Therefore a man must not trust the suggestions and illusions of his own heart.

The heart - There is nothing so false and deceitful as the heart of man; deceitful in its apprehensions of things, in the hopes and promises which it nourishes, in the assurances that it gives us; unsearchable by others, deceitful with reference to ourselves, and abominably wicked, so that neither can a man know his own heart, nor can any other know that of his neighbour's.