What does Psalm 33:21 mean?

Since the people of the congregation (Psalm 33:1–3) trust God for guidance and protection (Psalm 33:18–20), they can truly rejoice. Their hearts are glad in Him. Psalm 33 began an appeal for believers to shout for joy in the Lord. It ends with worshipers rejoicing in Him.

Worship should include heartfelt joy. Proper reflection on the Lord's character and deeds increases the believer's joy. Most often, happiness occurs because of happy circumstances. No one is expected to be "happy" when they are persecuted or suffering. But even then, a believer can experience "joy." In a biblical context, joy stems from knowing the Lord is loving, faithful, and gracious. Our joy is linked to our trust in God, specifically our trust in His "name," referring to who He is in character and actions. Even trials cannot erase joy in the Lord.

In his letter to the Philippians the apostle Paul alludes often to his rejoicing in the Lord. Although he was chained to prison guards around the clock, he was more intricately linked to the Lord by faith. He found may reasons to be joyful despite his circumstances (Philippians 1:7, 12–18; 2:17–18; 3:1; 4:4–8). Jesus promised His disciples, "Until now you have asked nothing in my name. Ask, and you will receive, that your joy may be full" (John 16:24).