Charles H. Spurgeon on Psalms 89:35

Once have I sworn by my holiness. He lays here his holiness to pledge for the assurance of his promise, as the attribute most dear to him, most valued by him, as though no other could give an assurance parallel to it, in this concern of an everlasting redemption, which is there spoken of. He that swears, swears by a greater than himself. God having no greater than himself, swears by himself; and swearing here by his holiness seems to equal that single to all his other attributes, as if he were more concerned in the honour of it than of all the rest. It is as if he should have said, Since I have not a more excellent perfection to swear by than that of my holiness, I lay this to pawn for your security, and bind myself by that which I will never part with, were it possible for me to be stripped of all the rest. It is a tacit imprecation of himself, If I lie unto David, let me never be counted holy, or thought righteous enough to be trusted by angels or men. This attribute he makes most of. —Stephen Charnock.