Albert Barnes Commentary Psalms 21:1

"The king shall joy in thy strength, O Jehovah; And in thy salvation how greatly shall he rejoice!" — Psalms 21:1 (ASV)

The king shall joy in thy strength — King David, who had achieved the victory which he had desired and prayed for (Psalms 20:1–9). This is in the third person, but the reference is doubtless to David himself and is to be understood as his own language. If it is understood, however, as the language of “the people,” it is still an ascription of praise to God for his favor to their king. It seems better, however, to regard it as the language of David himself.

The word "strength" here implies that all the success referred to was to be traced to God. It was not by the prowess of a human arm; it was not by the valor or skill of the king himself; it was by the power of God alone.

And in thy salvation — In the salvation or deliverance from foes which you have granted, and in all that you do to save. The language would embrace all that God does to save his people.

How greatly shall he rejoice! — Not only does he rejoice now, but he will always rejoice. It will be to him a constant joy. Salvation, now to us a source of comfort, will always be such; and when we once have evidence that God has interposed to save us, it is accompanied with the confident anticipation that this will continue to be the source of our highest joy forever.