Complete Commentary on the Whole Bible, by Matthew Henry, at sacred-texts.com
(Psa 21:1-6) Thanksgiving for victory.
(Psa 21:7-13) Confidence of further success.
Happy the people whose king makes God's strength his confidence, and God's salvation his joy; who is pleased with all the advancements of God kingdom, and trusts God to support him in all he does for the service of it. All our blessings are blessings of goodness, and are owing, not to any merit of ours, but only to God's goodness. But when God's blessings come sooner, and prove richer than we imagine; when they are given before we prayed for them, before we were ready for them, nay, when we feared the contrary; then it may be truly said that he prevented, or went before us, with them. Nothing indeed prevented, or went before Christ, but to mankind never was any favour more preventing than our redemption by Christ. Thou hast made him to be a universal, everlasting blessing to the world, in whom the families of the earth are, and shall be blessed; and so thou hast made him exceeding glad with the countenance thou hast given to his undertaking, and to him in the prosecution of it. The Spirit of prophecy rises from what related to the king, to that which is peculiar to Christ; none other is blessed for ever, much less a blessing for ever.
The psalmist teaches to look forward with faith, and hope, and prayer upon what God would further do. The success with which God blessed David, was a type of the total overthrow of all Christ's enemies. Those who might have had Christ to rule and save them, but rejected him and fought against him, shall find the remembrance of it a worm that dies not. God makes sinners willing by his grace, receives them to his favour, and delivers them from the wrath to come. May he exalt himself, by his all-powerful grace, in our hearts, destroying all the strong-holds of sin and Satan. How great should be our joy and praise to behold our Brother and Friend upon the throne, and for all the blessings we may expect from him! yet he delights in his exalted state, as enabling him to confer happiness and glory on poor sinners, who are taught to love and trust in him.