Psalms 89:35

Once have I sworn by my holiness,.... Swearing is ascribed to God after the manner of men, and is done in condescension to the weakness of his people, and to remove doubts and hesitations from them, relating to things spiritual and eternal; as to his everlasting love to them, his covenant with them in Christ, and their perseverance in his grace; and it is made by himself, or one or other of his perfections, as here by his "holiness"; see Amos 4:2, and indeed his holiness being his nature, is no other than he himself, the holy God; and because he could swear by no greater, he sware by himself, that as sure as he was, and was holy, just, and true, he would make good what he promises, Hebrews 6:13 and this is done but once, once for all, that being sufficient; it need not be repeated, nor is it ever revoked; when he swears, he never repents of it, nor changes his mind; and it is to show the immutability of his counsel that he swears at all,

Psalms 110:4,

that I will not lie unto David; he will not lie to any, he cannot, it is impossible he should; it would be to deny himself, it is contrary to his being as God, he is not a man that he should lie; it is contrary to his character as the God of truth; he will not lie, neither in his counsel nor covenant, in his purposes nor promises; these are the two immutable things, in which it is impossible he should lie: and he has swore to it that he will not lie to David, to David's son the Messiah, with whom the covenant is made, and stands fast; all the prophecies concerning him he has fulfilled; and all the promises made to him of help and assistance, as man and Mediator, in his work, and of the reward of it, a glory with him, he has made good.