They have set their mouth against the heavens. Here it is declared that they utter their contumelious speeches as well against God as against men; for they imagine that nothing is too arduous for them to attempt, and flatter themselves that heaven and earth are subject to them. If any should endeavor to alarm them by setting before them the power of God, they audaciously break through this barrier; and, with respect to men, they have no idea of any difficulty arising from such a quarter. Thus, there is no obstacle to repress their proud and vaunting speeches, but their tongue walketh through the whole earth. This form of expression seems to be hyperbolical; but when we consider how great and unbounded their presumption is, we will admit that the Psalmist teaches nothing but what experience shows to be matter of fact.

They set their mouth against the heavens - Compare Revelation 13:6. Literally, "They set their mouth in heaven," or in the heavens. The idea is, they speak as if they were "in" the heavens; as if they were clothed with all authority; as if they were superior beings, and had a right to command the universe.
And their tongue walketh through the earth - It has no limit; it is as if it roamed over all the earth. They speak without any restraint of law, or propriety; without any regard to the command of God, or to what is due to people, In other words, they seem to set themselves above all law, and to act as if there were no one in heaven or in earth to control them.

Set their mouth against the heavens - They blaspheme God, ridicule religion, mock at Providence, and laugh at a future state.
Their tongue walketh through the earth - They find fault with every thing; they traduce the memory of the just in heaven, and ridicule the saints that are upon earth. They criticize every dispensation of God.

They (d) set their mouth against the heavens, and their tongue walketh through the earth.
(d) They blaspheme God, and do not fear his power and rail upon men, because they esteem themselves above all others.

They set their mouth against the heavens,.... Against God in heaven, see Daniel 4:26, against his being, saying, there is no God; against his perfections, thinking him to be such an one as themselves; against his purposes and decrees, replying against him, and charging him with insincerity, cruelty, and unrighteousness; and against his providence, either denying it, or affirming it to be unequal; and against his doctrines, ordinances, and ministers. Aben Ezra interprets it also of the angels of heaven, who are spoken against, when it is denied that there are any such beings, as were by the Sadducees; and blasphemed, when the worshipping of them is introduced. The Targum understands it of the saints of heaven, with which compare Revelation 13:6 it may be applied to civil magistrates, the higher powers, who represent on earth God in heaven; and there are some that despise dominion, and speak evil of such dignities:
and their tongue walketh through the earth: sparing none, high nor low, but injures all sorts of persons with their lies and calumnies. This denotes the unbridled liberty which wicked men take with their tongues; there is no restraint upon them, no stopping of them; see Psalm 12:5 the universal mischief they are continually doing, and the diabolical influence of their detraction and falsehood; like Satan, their tongues walk to and fro in the earth, doing all the injury to the credit and characters of men they possibly can.

Against - Against God, blaspheming his name, and deriding his providence. Walketh - Using all manner of liberty, reproaching all sorts of persons.