Psalms 89:35 - Matthew Poole's English Annotations on the Holy Bible

Here he gives some reasons why he would not break his covenant with David, though he should have just cause to do so, and though he had upon such just cause broke his covenant made with others; first, Because this covenant was confirmed by his oath, which adds not only more solemnity, but more stability and certainty to it, as is evident from Hebrews 6:17, wherein he showeth that God addeth an oath to his promise or covenant to make and prove it to be immutable; and from Hebrews 7:20, &c., where he proveth the priesthood of Melchisedec to be unchangeable, because it was confirmed by an oath. And though judgments simply threatened have not always been executed, but sometimes were prevented, yet those comminations which were confirmed by oath were thereby rendered and declared to be irrevocable, as we see, Numbers 14:28-30 Jeremiah 44:26. Secondly, Because this is said to be sworn once, which word and phrase sometimes implies the completeness, certainty, and irrevocableness of the thing said or done, as Proverbs 28:18, shall fall at once. Thus Christ is said to have died or suffered once, Romans 6:10 Hebrews 9:26,28. Thirdly, Because God sware by his holiness; in or by which God is but seldom read to speak or swear, and when he is, it constantly adds more weight and confirmation to the speech, as Psalms 60:6, Psalms 108:7 Amos 4:2.