What does Psalm 56:9 mean?

Conviction that God was on his side reassured David. He was running from the jealousy of king Saul (1 Samuel 19:1–2) and was living among Philistine enemies (1 Samuel 21:11–15). Because he trusted deeply in the Lord's protection (Psalm 56:4), David confidently prayed for the Lord to move against his enemies (Psalm 56:7). Despite experiencing hardship and sorrow, David knew that his struggles were meaningful. God knew his troubles intimately (Psalm 56:8; Hebrews 4:15–16). Because the Lord cares for His people, David could confidently say, "God is for me," meaning He was on David's side.

Prayer is powerful. Second Corinthians 10:4 states, "For the weapons of our warfare are not of the flesh but have divine power to destroy strongholds." When Gideon battled the hordes of Midianites, his fighting force was comically outnumbered but Gideon was confident (Judges 7:15). His battle cry was "A sword for the Lᴏʀᴅ and for Gideon" (Judges 7:20). The Midianites panicked and fled. When Hezekiah prayed for Jerusalem's deliverance from the huge Assyrian army, the Lord answered by eliminating 185,000 Assyrians in their camp. Sennacherib and the survivors hurried home (2 Kings 19:14–36).

The apostle Paul assures believers in Romans 8:31 that "if God is for us, who can be against us?" He points out that in all things, "we are more than conquerors through him who loved us" (Romans 8:37). This does not mean that believers are immune to tragedy (John 16:33), but that God never allows us to experience anything without purpose.