Lest my enemy. David again repeats what he had a little before said concerning the pride of his enemies, namely, how it would be a thing ill becoming the character of God were he to abandon his servant to the mockery of the ungodly. David's enemies lay, as it were, in ambush watching the hour of his ruin, that they might deride him when they saw him fall. And as it is the peculiar office of God to repress the audacity and insolence of the wicked, as often as they glory in their wickedness, David beseeches God to deprive them of the opportunity of indulging in such boasting. It is, however, to be observed, that he had in his conscience a sufficient testimony to his own integrity, and that he trusted also in the goodness of his cause, so that it would have been unbecoming and unreasonable had he been left without succor in danger, and had he been overwhelmed by his enemies. We can, therefore, with confidence pray for ourselves, in the manner in which David here does for himself, only when we fight under the standard of God, and are obedient to his orders, so that our enemies cannot obtain the victory over us without wickedly triumphing over God himself.

Lest mine enemy say, I have prevailed against him - I have overpowered him; I have conquered him. That is, to triumph over him as having obtained a complete victory.
And those that trouble me - Hebrew, "My adversaries." The reference here is the same as in the former member of the verse. It is to the enemies that seemed almost to have triumphed over him already, and under whose power he was ready to sink. "Rejoice." Exult; triumph.
When I am moved - Moved from my steadfastness or firmness; when I am overcome. Hitherto he had been able to hold out against them; now he began to despair, and to fear that they would accomplish their object by overcoming and subduing him. His ground of apprehension and of appeal was, that by his being vanquished the cause in which he was engaged would suffer, and that the enemies of religion would triumph.

Let mine enemy say - Satan's ordinary method in temptation is to excite strongly to sin, to blind the understanding and inflame the passions; and when he succeeds, he triumphs by insults and reproaches. None so ready then to tell the poor soul how deeply, disgracefully, and ungratefully it has sinned! Reader, take heed.
When I am moved - When moved from my steadfastness and overcome by sin. O what desolation is made by the fall of a righteous soul! Itself covered with darkness and desolation, infidels filled with scoffing, the Church clad in mourning, the Spirit of God grieved, and Jesus crucified afresh, and put to an open shame! O God, save the pious reader from such wreck and ruin!

Lest mine enemy say, I have (c) prevailed against him; [and] those that trouble me rejoice when I am moved.
(c) Which might turn to God's dishonour: if he did not defend his.

Lest mine enemy say, I have prevailed against him,.... Which is an argument God takes notice of; and for which reason he does not give up his people into the hands of their enemies; see Deuteronomy 32:27. The Chaldee paraphrase interprets this of the evil imagination or corruption of nature, and represents it as a person, as the Apostle Paul does in Romans 7:15; and which may be said to prevail, when it pushes on to sin, and hinders doing good, and carries captive; and it may be applied to Satan, the great enemy of God's people, who triumphs over them, when he succeeds in his temptations;
and those that trouble me rejoice when I am moved; meaning from his house and family, from his country and kingdom, from a prosperous state and condition to a distressed one; at which the troublers of David's peace would rejoice. They that trouble the saints are sin, Satan, and the world; and the two last rejoice when they are in an uncomfortable and afflicted condition; and especially Satan rejoices when he gains his point, if it is but to move them from any degree of steadfastness, of faith and hope, or from the ways of God in any respect: the Targum adds, "from thy ways"; for to be moved so as to perish eternally they cannot, being built upon the Rock of ages, and surrounded by the power and grace of God.

rejoice--literally, "shout as in triumph."
I am moved--cast down from a firm position (Psalm 10:6).