To the Chief Musician. A Psalm of David.
1 The king shall have joy in Your strength, O Lord;
And in Your salvation how greatly shall he rejoice!
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 the blessings of goodness;
You set a crown of pure gold upon his head.
4 He asked life from You, and You gave it to him—
Length of days forever and ever.
5 His glory is great in Your salvation;
Honor and majesty You have placed upon him.
6 For You have made him most blessed forever;
You have made him exceedingly glad with Your presence.
7 For the king trusts in the Lord,
And through the mercy of the Most High he shall not be moved.
8 Your hand will find all Your enemies;
Your right hand will find those who hate You.
9 You shall make them as a fiery oven in the time of Your anger;
The Lord shall swallow them up in His wrath,
And the fire shall devour them.
10 Their offspring You shall destroy from the earth,
And their descendants from among the sons of men.
11 For they intended evil against You;
They devised a plot which they are not able to perform.
12 Therefore You will make them turn their back;
You will make ready Your arrows on Your string toward their faces.
13 Be exalted, O Lord, in Your own strength!
We will sing and praise Your power.
Where Psalm 20 pleads for victory, Psalm 21 celebrates it, although the victory may still be future (v.8). The connection can be seen by comparing Psalm 20:4 with Psalm 21:2.
The psalm contains two sections. The first, verses 1-7; the second, verses 8-13. In the first section, Jehovah is addressed; in the second, Messiah is addressed. The speaker in the psalm is Israel (v.13). The occasion intended in verses 1-7 is the morning of Messiah’s resurrection, and in verses 8-13 the morning of Israel’s restoration. — Williams, page 312.
selah (v.2) — a suspension of music, a pause
With respect to (Hebrew) grammar of verse 7, the first clause represents the basis of the second clause — i.e., because the aforementioned king trusts in the Lord, therefore the lovingkindness (i.e., the irrevocable paternal solicitude) of the Most High will be applied to him and he will not be shaken — referring to the eternal state in God’s presence. — Wechsler, page 71
Most High (v.7) — This title belongs to Melchizedek’s God (Genesis 14:18) whom Abram identified as Yahweh (Genesis 14:22) … As a title it signifies supreme dignity, unhampered power and universal sway. — Guthrie, page 464.
fiery oven (v.9) — The notion of God’s sovereignty in maintaining His promised solicitude towards His faithful is also intimated by the expression “fiery oven,” which recalls the phraseology of Genesis 15:17, in which God sovereignly ratifies His unconditional promise to Abraham. — Wechsler, page 71.
turn their back (v.12) = retreat, flee
in Your own strength (v.13) — The expression … here serves as a bookend with the same phrase at the beginning of this psalm, and underscored the true (and only enduring) source of strength of the godly king. — Wechsler, page 71
I am no Hebrew or Old Testament scholar, and I’m new to the study of the Psalms. But the Messianic theme of this psalm seems obvious to me, and I am amazed that only two of my nine commentaries mention it. The conclusion of Williams (quoted above) makes by far the most sense to me.