The enemy shall not exact upon him - None of his enemies shall be able to prevail against him. It is worthy of remark that David was never overthrown; he finally conquered every foe that rose up against him. Saul's persecution, Absalom's revolt, Sheba's conspiracy, and the struggle made by the partisans of the house of Saul after his death, only tended to call forth David's skill, courage, and prowess, and to seat him more firmly on his throne. The Philistines, the Ammonites, the Syrians, etc., united all their forces to crush him, but in vain: "God beat down all his foes before his face," and variously plagued those who opposed him, Psalm 89:23.
The enemy shall not exact upon him - The literal meaning here is derived from the force sometimes used in extorting or demanding a debt, where no indulgence is shown, but where it is exacted to the last mite, whether the man is able to pay it or not. Compare Matthew 18:25, Matthew 18:28. Then it is used to denote oppression, or subjugation, which is the idea here. The enemy shall not be suffered to act the part of one who rigidly exacts the payment of a debt; that is, he shall not be allowed to oppress him.
Nor the son of wickedness afflict him - This is copied almost literally from 2 Samuel 7:10. The phrase “the son of wickedness” means simply the wicked. He shall not fall into the hands, or under the power of wicked men.