My shield It is not wonderful that David often mingles meditations with his prayers, thereby to inspire himself with true confidence. We may go to God in prayer with great alacrity; but our fervour, if it does not gather new strength, either immediately fails or begins to languish. David, therefore, in order to continue in prayer with the same ardour of devotion and affection with which he commenced, brings to his recollection some of the most common truths of religion, and by this means fosters and invigorates his faith. He declares, that as God saves the upright in heart, he is perfectly safe under his protection. Whence it follows, that he had the testimony of an approving conscience. And, as he does not simply say the righteous, but the upright in heart, he appears to have an eye to that inward searching of the heart and reins mentioned in the preceding verse.

My defense is of God - The meaning here is, that God was his protector, and that in his troubles he confided in him. The original word here, as in Psalm 3:3, note; Psalm 5:12, note; is "shield." See the notes at thoses verses.
Which saveth the upright in heart - whom he that searches the heart Psalm 7:9 sees to be upright; or to be sincere, truthful, just. The writer says that it is a characteristic of God that he saves or protects all such; and, conscious of his innocence of the charges against himself, he here appeals to him on that ground, and confides in his protection because he sees that in this respect he was blameless.

My defense is of God - I now leave my cause in the hands of my Judge. I have no uneasy or fearful apprehensions, because I know God will save the upright in heart.

My defence is of God,.... Or "my shield is in" or "of God" (e); God was his shield, his protector and defender; see Psalm 3:3; or "my shield is with God"; that is, Christ, who was the shield his faith made use of against every spiritual enemy, was with God; he was with him as the Word and Son of God from all eternity, and as the living Redeemer of his people before his incarnation; and he is now with him as their intercessor and advocate, who pleads in defence of them, and opposes himself, his blood and righteousness, to all the charges and accusations of Satan;
which saveth the upright in heart: who have the truth of grace in them, wisdom in the hidden part; who are sincere in their affections, purposes, and designs, in their faith, hope, and love; and act from real principles of truth and love, in the integrity of their souls; for these light and gladness are sown, to them grace and glory are given, and no good thing is withheld from them; they are saved by God from sin, Satan, the world, death, and hell, and every enemy, with a spiritual and everlasting salvation.
(e) "in Deo", Musculus, Tigurine version, Junius & Tremellius, Muis, Ainsworth; "apud Deum", Lutherus, Piscator, Gejerus, Cocceius, Michaelis.

David is confident that he shall find God his powerful Saviour. The destruction of sinners may be prevented by their conversion; for it is threatened, If he turn not from his evil way, let him expect it will be his ruin. But amidst the threatenings of wrath, we have a gracious offer of mercy. God gives sinners warning of their danger, and space to repent, and prevent it. He is slow to punish, and long-suffering to us-ward, not willing that any should perish. The sinner is described, Psalm 7:14-16, as taking more pains to ruin his soul than, if directed aright, would save it. This is true, in a sense, of all sinners. Let us look to the Saviour under all our trials. Blessed Lord, give us grace to look to thee in the path of tribulation, going before thy church and people, and marking the way by thine own spotless example. Under all the persecutions which in our lesser trials mark our way, let the looking to Jesus animate our minds and comfort our hearts.

defence--literally, "shield" (Psalm 5:12).