C.H. Spurgeons's The Treasury of David
Verse 35. Once have I sworn by my holiness that I will not lie unto David. Because he could swear by no greater he swore by himself, and by that peculiar attribute which is his highest glory, being the subject of threefold adoration by all the hosts of heaven. God here pledges the crown of his kingdom, the excellent beauty of his person, the essence of his nature. He does as good as say that if he ceases to be true to his covenant he will have forfeited his holy character. What more can he say? In what stronger language can he express his unalterable adherence to the truth of his promise? An oath is the end of all strife; it ought to be the end of all doubt on our part. We cannot imagine that God could lie, yet he puts it so -- that if the covenant were not kept by him, he would regard it as a lie. Here is ground for strong confidence; may our faith be of such a nature as these assurances will warrant.
EXPLANATORY NOTES AND QUAINT SAYINGS
Verse 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.