What does Psalm 18:25 mean?

In this statement, David explains that some parts of our relationship with the Lord are reciprocal. If we are merciful to others, the Lord will be merciful to us. In celebrating God's mercy to him, David might have recalled how he showed mercy to his own enemies. David had shown mercy to his persecutor, Saul, by sparing his life (1 Samuel 24:4–11; 26: 1–25), and in turn the Lord extended mercy to David. God rescued David from Saul (2 Samuel 22:1) and made him king of Israel. David had been loyal to the Lord, and the Lord was faithful to David.

Though we clearly cannot earn our eternal salvation through our own good works or supposed righteousness (Titus 3:5; Romans 11:6; Ephesians 2:8–10), God does respond to our actions. Galatians 6:7 assures us that "whatever one sows, that will he also reap." Matthew 5:7 states: "Blessed are the merciful, for they shall receive mercy." Christ's model prayer for us contains a reciprocal relationship: we are to ask God to "forgive us our debts, as we also have forgiven our debtors" (Matthew 6:12). Jesus likewise says "if you forgive others their trespasses, your heavenly Father will also forgive you, but if you do not forgive others their trespasses, neither will your Father forgive your trespasses" (Matthew 6:14–15).