Psalms 21:7

The psalmist, having taught his people to look back with joy and praise on what God had done for him and them, here teaches them to look forward with faith, and hope, and prayer, upon what God would further do for them: The king rejoices in God (Psa 21:1), and therefore we will be thankful; the king trusteth in God (Psa 21:7), therefore will we be encouraged. The joy and confidence of Christ our King is the ground of all our joy and confidence. I. They are confident of the stability of David's kingdom. Through the mercy of the Most High, and not through his own merit or strength, he shall not be moved. His prosperous state shall not be disturbed; his faith and hope in God, which are the stay of his spirit, shall not be shaken. The mercy of the Most High (the divine goodness, power, and dominion) is enough to secure our happiness, and therefore our trust in that mercy should be enough to silence all our fears. God being at Christ's right hand in his sufferings (Psa 16:8) and he being at God's right hand in his glory, we may be sure he shall not, he cannot, be moved, but continues ever. II. They are confident of the destruction of all the impenitent implacable enemies of David's kingdom.

— Matthew Henry's Concise Commentary (public domain)

The mediate cause is the king's faith, the efficient, God's mercy.

— Jamieson-Fausset-Brown Commentary (public domain)