In Psalm 21, David praises Jehovah for his kingship and speaks of God's furious dealing with his enemies. David praised Jehovah for his kingship through His strengthening, salvation, blessings, crowning, lengthening of life, great glory, splendor and honor, and lovingkindness (vv. 1-7). David did not mention his righteousness here. If we depend upon our righteousness, our kingship can never be built up. Endeavoring to keep the law makes us slaves, not kings. According to Galatians 4, all the children brought forth by the law are slaves (vv. 24-25). But the children brought forth by grace become kings. To depend upon God's lovingkindness builds up the kingship.
In verses 8-12, David spoke of God's furious dealing with David's enemies by devouring them and destroying their descendants. This is David's concept. For his kingship he depended upon God's lovingkindness. For his enemies, however, he applied God's furious dealing.
In verse 13 David blessed Jehovah that He may be exalted in His strength that David as the king with his people will sing and psalm His might. This verse shows us a man who not only blesses his people but also blesses God. As a king, he blessed his people. As the one anointed by God, he blessed the anointing God. The conclusion of Psalm 21 is very good.
We can see much improvement with David in these five psalms. He has been improving gradually from Psalm 17 to the end of Psalm 21. This leads us to Psalm 22 concerning Christ's death, Psalm 23 concerning Christ's shepherding in His resurrection, and Psalm 24 concerning Christ's reigning as the coming King.