John Calvin Commentary Psalms 21:5

"His glory is great in thy salvation: Honor and majesty dost thou lay upon him." — Psalms 21:5 (ASV)

His glory is great. By these words, the people indicate that their king would become more renowned through the protection God provided him and the deliverances God accomplished for him, than if he had reigned in peace with universal applause, been defended by human wealth and human strength, or, finally, remained invincible by his own power and strategy. For by this, it appeared more clearly that he had attained the royal dignity only by the favor, guidance, and command of God.

Therefore, the believing Israelites leave it to pagan kings to exalt themselves by their own achievements and to acquire fame by their own valor. They set more value on God graciously showing favor to their king than on all the triumphs of the world.

At the same time, they expect such assistance from God as will be sufficient to adorn the king with majesty and honor.