John Calvin Commentary Psalms 13:4

"Lest mine enemy say, I have prevailed against him; [Lest] mine adversaries rejoice when I am moved." — Psalms 13:4 (ASV)

Lest my enemy. David again repeats what he had previously said concerning the pride of his enemies: namely, that it would be unfitting for God's character if He were to abandon His servant to the mockery of the ungodly. David’s enemies were, as it were, lying in ambush, watching for the moment of his ruin, so that they might deride him when they saw him fall. And since it is the unique role of God to repress the audacity and insolence of the wicked whenever they glory in their wickedness, David implores God to deny them the opportunity for such boasting.

It must be observed, however, that David had in his conscience a sufficient testimony to his own integrity, and he also trusted in the goodness of his cause. Consequently, it would have been unfitting and unreasonable if he had been left without help in danger and overwhelmed by his enemies. Therefore, we can confidently pray for ourselves, as David does here for himself, only when we fight under the standard of God and are obedient to His commands, so that our enemies cannot achieve victory over us without also wickedly triumphing over God Himself.