Thou hast trodden under foot all those who wander from thy statutes. By treading under foot he means, that God overthrows all the despisers of his law, and casts them down from that loftiness which they assume to themselves. The phrase is directed against the foolish, or rather frantic, confidence with which the wicked are inflated, when they recklessly deride the judgments of God; and, what is more, scruple not to magnify themselves against him, as if they were not subject to his power. The last clause is to be particularly noticed: for their deceit is falsehood [1] By these words the prophet teaches, that the wicked gain nothing by their wiles, but that they are rather entangled in them, or at length discover that they were mere sleight of hand. Those ignorantly mar the sense who interpose the copula and, as if it had been said, that deceit and falsehood were in them The word rmvh, remyah, signifies a subtle and crafty device. Interpreters, indeed, often translate it thought; but this term does not sufficiently express the propriety and force of the Hebrew word. The prophet means, that, however well pleased the wicked are with their own cunning, they yet do nothing else than deceive themselves with falsehood. And it was needful to add this clause; for we see how the great bulk of mankind are fatally intoxicated with their own vain imaginations, and how difficult it is to believe what is here asserted, -- that the more shrewd they are in their own estimation, the more do they deceive themselves.

Footnotes

1 - Dimock thinks that, by this expression, the Psalmist; probably alludes to the Lex Talionis amongst the Jews, and that the Apostle might refer to this passage in 2-Thessalonians 2:11; where he says, "that God should send them strong delusion, that they should believe a lie."

Thou hast trodden down all them that err from thy statutes - Compare the notes at Psalm 119:21. Rather, "Thou hast made light of," or "thou despisest." The Hebrew word means properly to suspend in a balance; to weigh. Then it means to lift up lightly or easily; and then, to make light of; to contemn; to regard anything as "light." The Septuagint and Latin Vulgate render it, "Thou dost despise." That is, God regards them as of no account; as a light substance of no value; as chaff which the wind carries away. Compare Job 21:18; Psalm 1:4; Psalm 35:5; Isaiah 17:13.
For their deceit is falsehood - This seems to be a truism - for deceit must imply falsehood. In the original this is an emphatic way of declaring the whole thing to be false, as the Hebrew language often expresses emphasis by mere repetition - thus "pits, pits," meaning many pits. The psalmist first characterizes their conduct as deceitful - as that which cannot be relied on - as that which must fail in the end; he then speaks of this system on which they acted as altogether a "lie" - as that which is utterly "false;" thus giving, as it were, a double emphasis to the statement, and showing how utterly delusive and vain it must be.

Thou hast trodden down - All thy enemies will be finally trodden down under thy feet.
Their deceit is falsehood - Their elevation is a lie. The wicked often become rich and great, and affect to be happy, but it is all false; they have neither a clean nor approving conscience. Nor can they have thy approbation; and, consequently, no true blessedness.

Thou hast trodden down all them that err from thy statutes: for their (d) deceit [is] falsehood.
(d) The crafty practises of them who contemn your law will be brought to nothing.

Thou hast trodden down all them that err from thy statutes,.... That wander from the way of the Lord's commandments; that deviate from his precepts, go astray constantly and wilfully; a people that err in their hearts, and with all their hearts. These the Lord treads down, as mire in the streets, as grapes in a winepress; which shows his abhorrence of them, his indignation at them, and how easily they are subdued under him;
for their deceit is falsehood: or, "their hypocrisy is a lie" (m): the appearance they make is a false one; they appear outwardly righteous, but are inwardly wicked; have a form of godliness, but deny the power of it: or all their deceitful doctrines are lies in hypocrisy, though dressed up with all the art and cunning they are masters of; or all their subtle schemes to corrupt and subvert the true doctrines of the word are in vain and to no purpose.
(m) So Michaelis.

But the disobedient and rebellious will be visited by God's wrath, which impresses the pious with wholesome fear and awe.
their deceit is falsehood--that is, all their cunning deceit, wherewith they seek to entrap the godly, is in vain.

Deceit - Their deceitful devices, shall bring that destruction upon themselves which they design for others.