John Calvin Commentary Psalms 54:5

"He will requite the evil unto mine enemies: Destroy thou them in thy truth." — Psalms 54:5 (ASV)

He shall reward evil unto mine enemies. Since the verb ישיב, yashib, can be translated he shall cause to return, it seems to indicate not only the punishment but also the kind of punishment that would be inflicted upon his enemies, as their wicked schemes recoil upon their own heads.

Some assign an optative meaning to the verb, understanding the words as expressing a wish or prayer. However, I see no reason why it should not be understood strictly in the future tense, and I believe David indicates his certain expectation that this favor, for which he had already prayed, would be granted.

It is quite common to find the Psalmist's prayers interspersed with sentences of this kind, inserted to stimulate his faith, as is the case here, where he declares the general truth that God is the righteous judge who will recompense the wicked.

To confirm his hopes, he refers particularly to the truth of God. For nothing can support us in the hour of temptation—when divine deliverance may be long delayed—except a firm conviction that God is true and that He cannot deceive us by His divine promises. His confidence in obtaining his request was grounded on the fact that God could no more deny His word than deny Himself.