Albert Barnes Commentary Psalms 5:9

"For there is no faithfulness in their mouth; Their inward part is very wickedness; Their throat is an open sepulchre; They flatter with their tongue." — Psalms 5:9 (ASV)

For there is no faithfulness in their mouth - There is nothing in them that can be trusted; nothing in their promises and declarations. They are false and treacherous, and I can, therefore, only appeal to you. It is easy to see the appropriateness of this statement, and of those that follow, assuming that this refers to the rebellion of Absalom. Absalom had gone to Hebron on a false pretense (2 Samuel 15:7–10), and every act of his in this whole transaction had been treacherous and false.

Their inward part - Not only their external conduct, but their hearts, their principles, their motives. This was fairly to be inferred from their conduct. The object of the psalmist is to show that they were wholly depraved in everything that properly constitutes character or that entered into moral conduct.

Their throat is an open sepulchre - That is, as the grave is open to receive its victim, so is their throat open to devour or swallow up the peace and happiness of others. The main idea is that they are false, treacherous, not to be trusted, slanderous. This passage, with the following, is used by the apostle Paul to demonstrate the universal depravity of man. See the notes at (Romans 3:13).

They flatter with their tongue - He had referred to the “inward part,” or the “heart,” and to the “throat” as being depraved and evil; he now refers to another member of the body as being equally depraved—the “tongue.” Instead of being used to utter truth, and to give expression to the real feelings of the heart, it was used to flatter others, in order to lead them astray, or to make use of them for base and selfish purposes. The appropriateness of this representation as applicable to Absalom and his associates no one can fail to see (Compare to 2 Samuel 15:1–6). It is also to a high degree the characteristic of the wicked in general. On this, also, see the notes at (Romans 3:13).