Once have I 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 unto David - Margin, as in Hebrew, “if I lie.” The meaning is, He would be found faithful to the promise. See Psalms 89:3-4; compare 2 Samuel 7:8-16.