Psalm 21 follows Psalm 20 both numerically and thematically. It is one distance removed from what happened in Psalm 20, but has the same basic themes: the king’s victory leads to joy and contemplation of more victories. I swipe the organization from Goldingay’s fine commentary (Psalms 1).
1. The King rejoicing (v. 1).
2. YHWH’s past deliverance (vv. 2-7)
3. YHWH’s future deliverance (vv. 8-12)
4. The People rejoicing (v. 13).
The priest, or someone, declares the reasons why the king can rejoice. “In your strength the king rejoices, O LORD, and in your help how greatly he exults!” (v. 1). The King can rejoice because of YHWH’s strength and help.
As Psalm 20 spoke of the King’s desire, so now the priest declares that God gave the King what he desired. He gets blessings and a gold crown; he gets long life and honor; he gets abundance and the very presence of YHWH. The priest now turns to why: because the King trusts YHWH and his commitment is for YHWH.
Psa. 21:0 To the leader. A Psalm of David.
1 In your strength the king rejoices, O LORD,
and in your help how greatly he exults!
2 You have given him his heart’s desire,
and have not withheld the request of his lips. Selah
3 For you meet him with rich blessings;
you set a crown of fine gold on his head.
4 He asked you for life; you gave it to him–
length of days forever and ever.
5 His glory is great through your help;
splendor and majesty you bestow on him.
6 You bestow on him blessings forever;
you make him glad with the joy of your presence.
7 For the king trusts in the LORD,
and through the steadfast love of the Most High he shall not be moved.
Psa. 21:8 Your hand will find out all your enemies;
your right hand will find out those who hate you.
9 You will make them like a fiery furnace
when you appear.
The LORD will swallow them up in his wrath,
and fire will consume them.
10 You will destroy their offspring from the earth,
and their children from among humankind.
11 If they plan evil against you,
if they devise mischief, they will not succeed.
12 For you will put them to flight;
you will aim at their faces with your bows.
Psa. 21:13 Be exalted, O LORD, in your strength!
We will sing and praise your power.