Albert Barnes Commentary Psalms 89:35

"Once have I sworn by my holiness: I will not lie unto David:" — Psalms 89:35 (ASV)

Once I have sworn by my holiness—That is, once for all; a single oath—an oath once taken by me—makes it certain. To swear by his “holiness” is to pledge his own holy nature; to make it as certain as that he is holy; to stake the whole question of his holiness on that. That is, if this should not be accomplished—if he should fail in this—it would prove that he was not a holy God.

That I will not lie to David—Margin, as in Hebrew, “if I lie.” The meaning is, He would be found faithful to the promise. ; (compare to 2 Samuel 7:8–16).