Psalm 21's title says it is for the Chief Musician and is a Psalm by David. Many readers pair it with Psalm 20: there the people prayed that God would grant the king his heart's desire; here they rejoice that he has. The king rejoices in your strength, Yahweh! How greatly he rejoices in your salvation! You have given him his heart's desire, and have not withheld the request of his lips.
God's gifts are listed with joy: blessings of goodness, a crown of fine gold, life and length of days forever and ever, glory, honor, and majesty. The greatest gift is in verse 6: You make him glad with joy in your presence. The psalm's hinge is verse 7: For the king trusts in Yahweh. Through the loving kindness of the Most High, he shall not be moved. The king's security rests on God's loving kindness, not on his armies.
Verses 8 to 12 turn to God's enemies. Using strong picture language, the psalm declares that God will defeat those who intended evil and plotted what cannot succeed. The final verse returns to worship: Be exalted, Yahweh, in your strength, so we will sing and praise your power. Believers read this as a reminder that the true King rules by God's power, and the New Testament presents Jesus as the King whose kingdom will have no end (Luke 1:33).