What does Psalm 18:47 mean?

Another reason David praised the living God is found here. God took vengeance on David's enemies and brought nations into subjection to him (2 Samuel 8:1–8, 14).

Although David was hunted and persecuted, he did not take vengeance into his own hands. He trusted the Lord to do what was right. For example, Saul was the prior king of Israel and one of David's greatest enemies. In a chance encounter, Saul became an easy target in the cave where David and his men were hiding. David's men urged him to kill Saul, but David refused to do so. He said, to his men, "The LORD forbid that I should do this thing to my lord, the LORD's anointed, to put out my hand against him, seeing he is the LORD's anointed" (1 Samuel 24:6).

In his letter to the Romans the apostle Paul counsels us to leave matters in the Lord's hands when we are maltreated. He quotes the Lord as saying, "Vengeance is mine, I will repay" (Romans 12:19).

David witnessed nations submitting to him. This reference likely has a messianic implication. When Jesus returns to earth, nations will submit to Him. In Revelation 2:26–27 Jesus promises to give authority to overcomers to rule the nations. He points out that the Father has given Him authority over the nations.