Matthew Poole and continuators on Psalms 89:35

35 Once have I sworn by my holiness , that I will not lye unto David.

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 onely more solemnity, but more stability and certainty to it, as is evident from Heb. 6. 17. wherein he sheweth that God added an Oath to his Promise or Covenant to make and prove it to be immutable; and from Heb. 7. 20, &c. where he proveth the Priesthood of Melchisedeck to be unchangeable, because it was confirmed by an Oath. And though judgments simply threatned have not always been executed, but sometimes were prevented, yet those comminations which were confirmed by Oath were thereby rendred and declared to be irrevocable, as we see Numb. 14. 28, 29, 30. Ier. 44. 26. Secondly, because this is said to be sworn once, which word and phrase sometimes implies the compleatness, certainty and irrevocableness of the thing said or done, as Prov. 28. 18. shall fall at once. Thus Christ is said to have died or suffered once, Rom. 6. 10. Heb. 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 Psal. 60. 6. and 108. 7. Amos 4. 2.

Marginal references: Heb. if I lie.