The point on which David insists so much is, that of showing from the effect or issue, that all his victories were to be traced to the favor of God; and from this it follows that his cause was good and just. God, no doubt, sometimes grants successes even to the ungodly and wicked; but he at length shows by the issue, that he was all the while opposed to them and their enemy. It is his servants alone who experience such tokens of his favor as he shewed towards David, and he intends by these to testify that they are approved and accepted by him. We are apt to think that David here speaks too much after the manner of a soldier, in declaring that he will not cease from the work of slaughter until he has destroyed all his enemies; or rather that he has forgotten the gentleness and meekness which ought to shine in all true believers, and in which they should resemble their heavenly Father; but as he attempted nothing without the command of God, and as his affections were governed and regulated by the Holy Spirit, we may be assured that these are not the words of a man who was cruel, and who took pleasure in shedding blood, but of a man who faithfully executed the judgment which God had committed to him. And, indeed, we know that he was so distinguished for gentleness of disposition as to abhor the shedding of even a single drop of blood, except in so far as duty and the necessity of his office required. We must, therefore, take into consideration David's vocation, and also his pure zeal, which was free from all perturbation of the flesh. Moreover, it should be particularly attended to that the Psalmist here calls those his enemies whose indomitable and infatuated obstinacy merited and called forth such vengeance from God. As he represented the person of Christ, he inflicted the punishment of death only on those who were so inflexible that they could not be reduced to order by the exercise of a mild and humane authority; and this of itself shows, that there was nothing in which he more delighted than to pardon those who repented and reformed themselves. He thus resembled Christ, who gently allures all men to repentance, but breaks in pieces, with his iron rod, those who obstinately resist him to the last. The sum of these verses is, that David, as he fought under the authority of God, being chosen king by him, and engaging in no undertaking without his warrant, was assisted by him, and rendered invincible against the assaults of all his enemies, and enabled even to discomfit vast and very powerful armies. Farther, let us remember, that under this type there is shadowed forth the invincible character and condition of the kingdom of Christ, who, trusting to, and sustained by, the power of God, overthrows and destroys his enemies, -- who, in every encounter, uniformly comes off victorious, -- and who continues king in spite of all the resistance which the world makes to his authority and power. And as the victories secured to him involve a security of similar victories to us, it follows that there is here promised us an impregnable defense against all the efforts of Satan, all the machinations of sin, and all the temptations of the flesh. Although, therefore, Christ can only obtain a tranquil kingdom by fighting, let us not on that account be troubled, but let it be enough to satisfy us, that the hand of God is always ready to be stretched forth for its preservation. David was, for a time, a fugitive, so that it was with difficulty he could save his life, by taking shelter in the dens of wild beasts; but God, at length, made his enemies turn their backs, and not only put them to flight, but also delivered them over to him, that he might pursue and utterly discomfit them. In like manner, our enemies for a time may be, as it were, just ready to put the knife to our throat to destroy us, but God, at length, will make them not only to flee before us, but also to perish in our presence, as they deserve. At the same time, let us remember what kind of warfare it is to which God is calling us, against what kind of persons he will have us to contend, and with what armor he furnishes us, that it may suffice us to have the devil, the flesh, and sin overthrown and placed under our feet by his spiritual power. With respect to those to whom he has given the power of the sword, he will also defend them, and not suffer them to be unrighteously opposed, provided they reign under Christ, and acknowledge him as their head. As to the words, interpreters almost unanimously render the beginning of the 40th verse, My enemies have turned the back, a phrase of the same import as, They have been put to flight; but as the Hebrew word rph, oreph, properly signifies the head or neck, we may very suitably view the words as meaning that God gave David the neck of his enemies, inasmuch as he delivered them into his hands to be slain.

I have pursued mine enemies, and overtaken them - He had not only routed them, but had had strength to pursue them; he had not only pursued them, but he had been enabled to come up to them. The idea is that of complete success and absolute triumph.
Neither did I turn again - I was not driven back, nor was I weary and exhausted, and compelled to give over the pursuit.
Till they were consumed - Until they were all either slain or made captive, so that the hostile forces vanished. None of my enemies were left.

(d) I have pursued mine enemies, and overtaken them: neither did I turn again till they were consumed.
(d) David declares that he did nothing besides his calling, but was stirred up by God's Spirit to execute his judgments.

I have pursued mine enemies, and overtaken them,.... Which may refer to David's pursuing the Amalekites, who overtook them and recovered all they had carried away, 1-Samuel 30:8; so Kimchi explains it;
neither did I turn again till they were consumed; for not a man escaped, save four hundred young men that rode on camels and fled, Psalm 18:17.

In actual conflict, with God's aid, the defeat of his enemies is certain. A present and continued success is expressed.

(Hebrews.: 18:38-41) Thus in God's strength, with the armour of God, and by God's assistance in fight, he smote, cast down, and utterly destroyed all his foes in foreign and in civil wars. According to the Hebrew syntax the whole of this passage is a retrospect. The imperfect signification of the futures in Psalm 18:38, Psalm 18:39 is made clear from the aorist which appears in Psalm 18:40, and from the perfects and futures in what follows it. The strophe begins with an echo of Exodus 15:9 (cf. supra Psalm 7:6). The poet calls his opponents קמי, as in Psalm 18:49, Psalm 44:6; Psalm 74:23, cf. קימנוּ Job 22:20, inasmuch as קוּם by itself has the sense of rising up in hostility and consequently one can say קמי instead of עלי קמים (קומים 2-Kings 16:7).
(Note: In the language of the Beduins kôm is war, feud, and kômānı̂ (denominative from kōm) my enemy (hostis); kōm also has the signification of a collective of kōmānı̂, and one can equally well say: entum waijânâ kôm, you and we are enemies, and: bênâtnâ kôm, there is war between us.)
The frequent use of this phrase (e.g., Ps 36:13, Lamentations 1:14) shows that קום in Psalm 18:39 does not mean "to stand (resist)," but "to rise (again)." The phrase נתן ערף, however, which in other passages has those fleeing as its subject (2-Chronicles 29:6), is here differently applied: Thou gavest, or madest me mine enemies a back, i.e., those who turn back, as in Exodus 23:27. From Psalm 21:13 (תּשׁיתמו שׁכם, Symm. τάξεις αὐτοὺς ἀποστρόφους) it becomes clear that ערף is not an accusative of the member beside the accusative of the person (as e.g., in Deuteronomy 33:11), but an accusative of the factitive object according to Ges. 139, 2.