Psalms 89:35

Once have I sworn by my holiness, &c. Here he assigns some reasons why he would not break his covenant with David, though he should have just cause so to do, and though he had, upon such just cause, broken his covenant made with others; 1st, Because this covenant was confirmed by his oath, which added, not only more solemnity, but more stability and certainty to it, according to the apostle's reasoning, Hebrews 6:13, whereby he shows that God added an oath to his promise made to Abraham, to make and prove it to be immutable; and Hebrews 7:20, &c., where he proves the priesthood of Melchisedek to be unchangeable, because it was confirmed by an oath. And although judgments, simply threatened, have not always been executed, but sometimes prevented; yet those comminations, which were confirmed by oath, were thereby rendered and declared to be irrevocable, as we see Numbers 14:28; Jeremiah 44:26. 2d, Because God sware by his holiness; in or by which he is but seldom said to speak or swear, and when he is, it constantly adds more weight and confirmation to what is declared, as Psalms 60:6; Psalms 108:7; Amos 4:2.