What does Psalm 35:9 mean?

David concludes his request for the destruction of his enemies (Psalm 35:1–8) by celebrating his relationship with God. He plans, in advance, to acclaim God and thank Him for being rescued. This shows that David fully believed the Lord would answer his prayer. In David's mind, what he asks for was as good as done.

In part, David was relying on the Lord's promise. God vowed that David would be king over Israel (1 Samuel 16:12–13). This psalm was probably written in response to David's persecution by the former king of Israel, Saul (1 Samuel 19:1–2). Because he rejected God's will and tried to overturn His predictions, Saul's plot to destroy David was doomed from the start.

Faith which rests on God's promises is never disappointed. Paul writes in 2 Corinthians 1:20: "For all the promises of God find their Yes in him. That is why it is through him that we utter our Amen to God for his glory." Paul also writes in Titus 1:2 that God never lies. Prayer that is based on selfish motives or sinful desires does not honor God; we cannot expect Him to listen to it (James 4:3; John 9:31; Isaiah 59:2; Psalm 66:18). But prayers based on God's promises honor Him; He hears our prayers and grants our requests when they are in accordance with His will (1 John 5:14; Luke 22:42). Notice, too, that prayer builds relationship. Even as David trusts that God will grant his request, he speaks of rejoicing in God. David does not ask for things of God as a type of cosmic vending machine, but out of relationship. He entrusts his heart to God. Often times, prayer changes our hearts and conforms our minds to God's truth.