Psalm 21 celebrates in vv. 8-12 the future delivering acts of YHWH. It is not entirely clear at times whether the speaker is speaking of the King or of YHWH, but the two are clearly interwoven, something that is both true of the Psalms and a potential source of concern in our world today.
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 furnacewhen 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.
But the celebration is directed at YHWH, not the king. This is seen in v. 13. It is YHWH who is on high. This psalm is used on the Feast of the Ascension, and the enemies defeated are seen as the powers and authorities. It fits perfectly, then, with 1 Corinthians 15 and the early Christian gospel of celebrating the victory of Christ in his resurrection and of Christ’s exaltation to the right hand of the Father.